What's going on here? מַה זֶה?

Our parashah introduces the י”ג מִדּוֹת הָרַחֲמִים (shelosh esrei middot ha-rahamim, 13 attributes of God’s mercy). We say these words a lot in Selihot prayers and on Yom Kippur.

What are they, and how do they work?

The Talmud says that God taught Moshe how to pray with the 13 middot in our parashah. God said, “Whenever Benei Yisrael sin, they should perform this arrangement before Me, and I will forgive them” (Rosh Hashanah 17b). In fact, later in the Torah, it works! In Parashat Shelah, Benei Yisrael sin again by believing the spies’ bad report about the Land of Israel, so Moshe prays using language from the 13 middot, and God forgives them (Bemidbar 14:18-20).
The Etz Yosef says that it’s not saying that really matters, it’s doing. We gain God’s forgiveness when we use the 13 middot in our own behavior, such as by making ourselves more compassionate, gracious, and slow to anger (Etz Yosef on Ein Yaakov Rosh Hashanah 17b).