What it does
Boast is an activated ability you can only use once each turn, and only if that creature attacked this turn. You pay the boast cost (mana and/or other requirements) any time after attacking, during a phase when you have priority, to get the listed effect.
The big nuance people miss: the creature only needs to have attacked—it doesn't need to still be on the battlefield, attacking, or have connected with a player. As long as it was declared as an attacker this turn, you can boast even if it's since been blocked, removed from combat, or the combat damage step has passed. You also can't boast on the turn the creature entered if it didn't attack.
In Commander, boast rewards aggression and gives you repeatable value engines across long games—just remember to leave mana up after declaring attackers, since you can't boast before combat.
