Walk-off means that the other team literally "walks-off" the field.
Usually, because there is no mercy rule in higher level baseball, it means when the winning run is up or on base and the batter hits a homerun in the bottom of the inning to win it (it can't be in the top, because the fielders don't "walk-off" because they get another chance to bat; when it happens in the top it's a go-ahead homerun.)