Exert allows creatures to give a little more effort to produce unusually good results.
You make the decision whether to exert a creature as you declare it as an attacker. If you choose to have it exert, an ability will trigger and grant you some bonus. As a trade-off, the creature won’t untap during your next turn. It’s tired. Needs a nap. Attack more later.
You don’t have to exert the creature as it attacks. If you don’t, no ability will trigger, and it will untap normal during your next turn. Note that as you declare attackers is the only time you can have it exert. You can’t wait until later in the turn and then exert it. When your next untap step rolls around, if the creature you exerted is untapped, nothing happens and the exertion cost you nothing. Maybe you found a different way to untap it, or maybe you gave it vigilance.
