Yeah, it means a one-piece advantage (and specifically, a "piece" in this context means a bishop or a knight) with everything else being equal. It's possible f

Yeah, it means a one-piece advantage (and specifically, a "piece" in this context means a bishop or a knight) with everything else being equal. It's possible for one side to have a crushing positional advantage that is worth more than a piece, but that's relatively rare. For 90% of quiescent positions reached by strong players where one side has the advantage, "count the pieces" is indeed enough to tell you which side it is.