Baseball is about scoring runs and preventing the other team from scoring runs, and everything a player does on a baseball field contributes (positively or negatively) to one of those two things. Of course, it’s not that simple; players can contribute to scoring runs in a variety of different ways. For instance, successfully stealing a base contributes to scoring a run. So does taking a walk, or hitting a home run. Likewise, a defender can contribute the other way with a particularly great outfield arm or by minimizing errors. Pretty straightforward.
Not everything a player does has the same value, which is also relatively obvious.