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 rank 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.
Let’s say you have Power 1 and your path lets you learn two spells. You can learn up to rank 1 spells. If you wanted to learn a rank 1 spell from the Air tradition, flensing wind for example, you would first have to learn the rank 0 spell, direct wind, from the Air tradition. Once you learn direct wind, whenever you learn a spell, you can freely learn spells from the Air tradition.
Once you expend the last casting of a spell, you cannot cast it again until you regain at least one casting for that spell. Talents from paths may allow you to regain castings during play, but everyone regains all expended castings after they complete a rest.