Firebending is a triggered ability found on attacking creatures. The number after the keyword tells you how much red mana you get: a creature with firebending 2 adds {R}{R} whenever it attacks, and that mana lasts until end of combat. You get the mana as the attack is declared, and it stays available through the combat phase. Any mana you do not spend before combat ends is lost.
That timing is what makes firebending useful. You can attack with your creature and still have mana open to cast an instant, activate an ability, or pay a cost during combat—after blockers are chosen, for example. It rewards attacking and lets aggressive red and black decks keep pressure on while holding up tricks. The bigger the number, the more explosive the turn.
