You’ll probably hear something about errors early on in learning the rules of baseball. An error is a mishap by a defensive player that gives the offense an advantage that they wouldn’t generally receive. There are quite a few ways that an error can be charged to a fielder, but most stem from it being a misplay (a fumble, muff, or wild throw as the rulebook puts it) that allows a runner to advance one or more bases, extends a batter’s time at the plate, or extends a runners time on the bases.
Generally errors occur when a fielder bobbles a ground ball or makes a poor throw that allows a batter to reach base. It can also happen with fly balls—lots of errors are committed when a fielder drops a ball he should have caught.
Some people might try to tell you that if a ball hits a fielder’s glove then it’s an error; don’t listen to them. The rules state that it is a play that should have been made by a fielder using “ordinary effort.” This is a judgment call that will be made by the official scorer. If a player chases down a pop fly running full speed and dives only to have the ball bounce in and out of his glove, he had demonstrated more than ordinary effort, and the play would likely be ruled a base hit.
Also keep in mind that the player must physically commit the error, if an infielder cleanly fields a ball and makes an accurate throw to first base that is beaten out by the runner it is a base hit as there were no fumbles, muffs, or wild throws on the play.