brendon described it good lol. lets say for example the ball is coming at you. you sort of catch it but u drop it or some shit. thats a error. or if you throw it way over someones head thats a error. heres what google says.
In baseball statistics, an error is an act, in the judgment of the official scorer, of a fielder misplaying a ball in a manner that allows a batter or baserunner to advance one or more bases or allows an at bat to continue after the batter should have been put out.