This is the number in the lower left corner of a creature card. A creature's power is how well it battles. When creatures battle, the one with the higher power defeats the other one. In a tie, they defeat each other.
Some creatures have plus signs (+) next to their power numbers. The plus sign is a reminder on creatures that sometimes get extra power from special abilities like "power attacker."
Other creatures have minus signs (-) next to their power numbers. The minus sign is to show that when a certain effect is triggered that the creatures sometimes lose power. When a monster like this has its power reach 0, it is destroyed.