Rampage is a triggered ability that cares about how many creatures block the same attacker. The reminder text reads: whenever this creature becomes blocked, it gets +N/+N until end of turn for each creature blocking it beyond the first. So a single blocker gives no bonus. If two creatures block it, it gets the bonus once; three blockers, twice, and so on. The size of the bonus is written on the card, such as rampage 1, 2, or 3.
In play, rampage makes blocking awkward for the defender. Gang-blocking a rampage creature to kill it can pump it enough to survive or trade up, so one big blocker is often better than several small ones. It shines on large red and green creatures that already hit hard, turning extra blockers into a liability. Trample pairs especially well, because the pumped power can spill over to the player.
