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.
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.