רַבָּן גַּמְלִיאֵל הָיָה אוֹמֵר, כָּל שֶׁלֹּא אָמַר שְׁלֹשָׁה דְבָרִים אֵלּוּ בְּפֶסַח, לֹא יָצָא יְדֵי חוֹבָתוֹ, וְאֵלּוּ הֵן, פֶּסַח, מַצָּה, וּמָרוֹר. פֶּסַח, עַל שׁוּם שֶׁפָּסַח הַמָּקוֹם עַל בָּתֵּי אֲבוֹתֵינוּ בְמִצְרַיִם. מַצָּה, עַל שׁוּם שֶׁנִּגְאֲלוּ אֲבוֹתֵינוּ בְמִצְרַיִם. מָרוֹר, עַל שׁוּם שֶׁמֵּרְרוּ הַמִּצְרִים אֶת חַיֵּי אֲבוֹתֵינוּ בְמִצְרָיִם. בְּכָל דּוֹר וָדוֹר חַיָּב אָדָם לִרְאוֹת אֶת עַצְמוֹ כְאִלּוּ הוּא יָצָא מִמִּצְרַיִם...
Rabban Gamliel used to say: Whoever does not mentioned these three things on Passover does not discharge his duty, and these are they: the Passover-offering, unleavened bread, and bitter herbs. [The] Passover-offering [is offered] because the Omnipresent One passed over the houses of our ancestors in Egypt. Unleavened bread [is eaten] because our ancestors were redeemed from Egypt. [The] bitter herb is [eaten] because the Egyptians embittered the lives of our ancestors in Egypt. In every generation a person must regard himself as though he personally had gone out of Egypt....
כל אחד מהמסבִים לוקח כזית מרור, ּמטבִלו בַחרוסת, ּמנער החרוסת, מברך ואוכל בלי הסבה.
All present should take a kazayit of marror, dip into the haroset, shake off the haroset, make the blessing and eat without reclining.
Blessed are You, Lord our God, King of the Universe, who has sanctified us with His commandments and has commanded us on the eating of marror.
הֵבִיאוּ לְפָנָיו, מְטַבֵּל בַּחֲזֶרֶת, עַד שֶׁמַּגִּיעַ לְפַרְפֶּרֶת הַפַּת. הֵבִיאוּ לְפָנָיו מַצָּה וַחֲזֶרֶת וַחֲרֹסֶת וּשְׁנֵי תַבְשִׁילִין, אַף עַל פִּי שֶׁאֵין חֲרֹסֶת מִצְוָה. רַבִּי אֱלִיעֶזֶר בְּרַבִּי צָדוֹק אוֹמֵר, מִצְוָה.
[Then] they set [food] before him. He dips the lettuce before he reaches the course following the [unleavened] bread. [Then] they set before him unleavened bread, lettuce, and a mixture of apples, nuts, and wine, and two dishes, although the mixture of apples, nuts, and wine is not compulsory. Rabbi Eliezer bar Tzadok says: It is compulsory.
צריך לשקועיה בחרוסת משום קפא דאי ס"ד לא צריך לשקועיה נטילת ידים למה לי הא לא נגע ודילמא לעולם אימא לך לא צריך לשקועיה וקפא מריחא מיית
one must submerge it in the ḥaroset, due to the bitterness and poison in the lettuce. For if it could enter your mind that one need not thoroughly submerge the lettuce in ḥaroset, why do I need him to wash hands before eating bitter herbs? He did not touch the liquid with his hands, and therefore he did not render it ritually impure. The Gemara rejects this contention: Actually, I will say to you: According to the halakha, one need not submerge the lettuce in ḥaroset and the poison dies. The poison is nullified from the smell of the ḥaroset.
צריך לשקועיה בחרוסת משום קפא - ארס שבחזרת שהשרף שבחזרת יש בו ארס כדרך הבצלים:
It should be immersed in the Charoset because of the "Kapa" - a venom/poison in the lettuce, since the burning effect of lettuce is the result of a venom/poison that needs to be neutralized, just like with onions.
אע"פ שאין חרוסת מצוה: ואי לא מצוה משום מאי מייתי לה א"ר אמי משום קפא אמר רב אסי קפא דחסא חמא קפא דחמא כרתי [קפא דכרתי חמימי] קפא דכולהו חמימי אדהכי והכי נימא הכי קפא קפא דכירנא לך ולשב בנתיך ולתמני כלתך:
The mishna states that they bring the ḥaroset to the leader of the seder, although eating ḥaroset is not a mitzva. The Gemara asks: And if it is not a mitzva, for what reason does one bring it to the seder? Rabbi Ami said: It is brought due to the poison in the bitter herbs, which is neutralized by the ḥaroset. In this regard, Rav Asi said: The remedy for one who ate the poison in lettuce is to eat a radish. The remedy for the poison in a radish is leeks. The remedy for the poison in leeks is hot water. A remedy for the poison in all vegetables is hot water. The Gemara comments: In the meantime, while one is waiting for someone to bring him the remedy, let him say the following incantation: Poison, poison, I remember you, and your seven daughters, and your eight daughters-in-law.
מִ֣י זֹ֗את עֹלָה֙ מִן־הַמִּדְבָּ֔ר מִתְרַפֶּ֖קֶת עַל־דּוֹדָ֑הּ תַּ֤חַת הַתַּפּ֙וּחַ֙ עֽוֹרַרְתִּ֔יךָ שָׁ֚מָּה חִבְּלַ֣תְךָ אִמֶּ֔ךָ שָׁ֖מָּה חִבְּלָ֥ה יְלָדַֽתְךָ׃
Who is she that comes up from the desert, Leaning upon her beloved? Under the apple tree I roused you; It was there your mother conceived you, There she who bore you conceived you.
צריך לסמוכיה וצריך לקהוייה - ובירושלמי אמר אית דעבדי זכר לדם ומשום הכי קרי ליה טיבולו במשקה וכן עמא דבר לסמוכי ובשעת אכילה מקלשין אותו ביין וחומץ ובתשובת הגאונים מפרש לעשות חרוסת בפירות שנדמה לכנסת ישראל בשיר השירים תחת התפוח עוררתיך כפלח הרמון התאנה חנטה אמרתי אעלה בתמר אגוז אל גנת אגוז ושקדים על שם ששקד הקב"ה על הקץ:
One must prepare it tart and one must prepare it thick- In the Yerushalmi it also says that it is supposed to be a remembrance of the blood, and therefore we call the act "dipping" like we call it with a liquid. This is a matter that the people follow, adding wine and vinegar at the time of eating to make the consistency more liquidy. I have found additional positions in the Gaonim which say that charoset should be made of the fruits that greeted the Israelites when they came into the land of Israel, as the Song of Songs mentions apples, pomegranate, fig, dates, nuts and almonds.
הַחֲרֹסֶת מִצְוָה מִדִּבְרֵי סוֹפְרִים זֵכֶר לַטִּיט שֶׁהָיוּ עוֹבְדִין בּוֹ בְּמִצְרַיִם. וְכֵיצַד עוֹשִׂין אוֹתָהּ. לוֹקְחִין תְּמָרִים אוֹ גְּרוֹגָרוֹת אוֹ צִמּוּקִין וְכַיּוֹצֵא בָּהֶן וְדוֹרְסִין אוֹתָן וְנוֹתְנִין לְתוֹכָן חֹמֶץ וּמְתַבְּלִין אוֹתָן בְּתַבְלִין כְּמוֹ טִיט בְּתֶבֶן
The charoset is a mitzvah ordained by the words of the Sages, to commemorate the clay with which [our forefathers] worked in Egypt. How is it made?
We take dates, dried figs, or raisins and the like, and crush them, add vinegar to them, and mix them with spices, as clay is mixed into straw.
Marbeh Lisaper on Pesach Haggadah, Magid, Four Questions 2:5 The second explanation of Mah Nishtanah is related to Rabban Gamliel’s statement later in the Haggadah in which he explains why we eat the Passover offering, the matzah and the bitter herbs. Clearly, the reason for matzah is to commemorate freedom while the reason for the bitter herbs is to remind us of slavery. We eat them together with the Passover offering to include all of these remembrances together.... therefore, the bitter herbs actually served the purpose of making Israel worthy of receiving the Torah – this happened after the Exodus so the question of bitter herbs also follows the question regarding the matzah.
The Mah Nishtanah implies this explanation. The child wants to know why we ask questions regarding freedom first before we ask questions about slavery. After all, matzah is a symbol of freedom while matzah and dipping are symbols of slavery. The dipping in charoset is a reminder of the mortar with which they made the bricks, while leaning is a symbol of freedom. Even though Don Isaac Abarbanel suggested that matzah is a symbol of slavery and we call it the bread of affliction, in reality it is a symbol of freedom; that is why we are supposed to lean when we eat the matzah. There is also an association of freedom with bitter herbs, thus while we don’t lean when we eat the bitter herbs, it is permissible to do so!