Tribute is a static ability introduced in Born of the Gods.  It gives opponents an unfortunate choice: have the creature grow in size, or allow a powerful enters-the-battlefield ability to trigger.
From the Comprehensive Rules (Born of the Gods, 2 Feb 2014):
- 702.103. Tribute
- 702.103a Tribute is a static ability that functions as the creature with tribute is entering the battlefield. "Tribute N" means "As this creature enters the battlefield, choose an opponent. That player may have this creature enter the battlefield with an additional N +1/+1 counters on it."
- 702.103b Objects with tribute have triggered abilities that check "if tribute wasn’t paid." This condition is true if the opponent chosen as a result of the tribute ability didn’t have the creature enter the battlefield with +1/+1 counters as specified by the creature’s tribute ability.
- Nessian Demolok -
Creature — Beast
Tribute 3 (As this creature enters the battlefield, an opponent of your choice may put 3 +1/+1 counters on it.)
When Nessian Demolok enters the battlefield, if tribute wasn't paid, destroy target noncreature permanent.</ref>