what does compraste mean?
You are exactly right. It indeed comes from the verb "comprar" (to buy).
Compraste is the second person singular, familiar form of the verb comprar in the preterit tense, indicative mood.
Compraste ---> you (familiar) bought
Girl speaking to her brother
You bought mom an iPad for Christmas
Le compraste a mamá un iPad para la Navidad.
I hope this helps!
It means "You bought". It is the preterit tense of "comprar" in the "tú" form--that is, if you want to be technical with your grammar .