When a Creature, whether the player's or opponent's, enters your side of the battlefield a second time from anywhere in the game, the base stats of the new Creature are added to the Creature already on the battlefield.

Example: Leaf Gilder is a 2/1 Creature. When a second Leaf Gilder is cast, the Leaf Gilder on the battlefield becomes a 4/2 and you will see a number that indicates how many times that card was reinforced (entered the battlefield).