Power does two things. First, it tells you the highest rank spell you can learn. (I’ll explain ranks below.) Second, it tells you how many castings of a spell you have for any given rank. At Power 1, you have 1 casting for all rank 1 spells you know. At Power 2, you have 2 castings of rank 1 spells and 1 casting of rank 2 spells. It goes on from there. A table in the Magic chapter shows you how castings increase. Rank 0 spells, minor spells, have unlimited castings.