According to the book of Jonah:
G‑d commanded Jonah to go to Nineveh, one of the largest cities of that time and prophecize its destruction, because the evil of its inhabitants had reached the limit. However, Jonah did not want to be prophet (a dangerous and often very difficult job), so he decided to run away from this responsibility. He boarded a ship to Tarshish (modern-day Turkiye).
But once they were on the high seas, G‑d caused a storm to break that threatened to tear the ship asunder. The sailors were frightened and each one began to pray. Jonah, however, lay down to sleep. The captain of the ship went over to him and reprimanded him for sleeping in that fateful hour, instead of praying to G‑d. Jonah revealed that the storm was his own fault, and advised the sailors to throw him overboard in order to save their own lives. The sailors reluctantly agreed, and tossed him down into the depths, where Jonah was then saved from drowning by being swallowed by a giant fish.
Important themes:
- G-d will protect and redeem us, even when we don't want it
- Even a whole population of evildoers can be redeemed if they atone; G-d does not hold grudges
- G-d wants us to choose good
Why do you think we read Jonah on Yom Kippur?
Perhaps it's because we hope for G-d to meet our aspirations. We hope that G-d will protect and redeem us, that G-d will not hold grudges against us for our mistakes. We want to believe that G-d has hope in us.
אָמַר לוֹ הַדָּג לְיוֹנָה, אִי אַתָּה יוֹדֵעַ שֶׁבָּא זְמַנִּי לְהֵאָכֵל לְתוֹךְ פִּיו שֶׁל לִוְיָתָן. אָמַר לוֹ: הוֹלִיכֵנִי לְשָׁם וַאֲנִי מַצִּיל אוֹתְךָ וְאֶת נַפְשִׁי. הוֹלִיכוֹ אֵצֶל לִוְיָתָן. אָמַר לוֹ לַלִּוְיָתָן, בִּשְׁבִילְךָ בָּאתִי לִרְאוֹת מְדוֹרְךָ בַּיָּם. וְלֹא עוֹד, אֶלָּא שֶׁאֲנִי עָתִיד לֵירֵד לִתֵּן חֶבֶל בְּצַוָּארְךָ וְלַעֲלוֹת אוֹתְךָ לִסְעוּדָה גְּדוֹלָה שֶׁל צַדִּיקִים. הֶרְאָהוּ חוֹתָמוֹ שֶׁל אַבְרָהָם אָבִינוּ, הִבִּיט לַבְּרִית וְרָאָהוּ לִוְיָתָן וּבָרַח מַהֲלַךְ שְׁנֵי יָמִים לִפְנֵי יוֹנָה.
The fish said to Yonah, "Do you not know that my time has come to be eaten into the mouth of the Leviathan?" He said to it, "Take me there and I will save you, and my soul." It took him to the Leviathan. He said to the Leviathan, "Because of you have I come to see your dwelling place in the sea. And not only that, but in the future I will come down to put a rope on your neck and to bring you up for the great meal of the righteous ones." He showed it his seal from Avraham, our father (his circumcision). The Leviathan saw it and fled the journey of two days from before Yonah.