+1 or -1. Well, that means: power or toughness of the creature is improved or decreased (depending) regardless of the actual power or toughness. For example, in the creatures with graft, they come into play 0/0. They would be removed to graveyard, but the ability "graft" allows come with X counters +1/+1. (Graft 1, graft 2, graft 3...). In the case of -1, is different. If the creature's toughness come to 0, the creature is removed from the field to the graveyard (normally). The creature isn't destroyed, simply that doesn't resist enough to life.