In baseball, there are two things a team or player can do to help win games: score runs and prevent runs. While this may seem ridiculously obvious, its simplicity is sometimes overlooked—as long as you are good enough at one of those two things, you can be below average at the other one.
The trick, of course, is that not everything on the field is equally important. For position players, for instance, hitting is more important than fielding; this is because you have a finite amount of outs to work with and avoiding outs is extremely important. This is why there are lots of great defenders who don’t stick in the big leagues: they just can’t hit.