Essentially, Exert is an extra ability that you can choose to use as attackers are being declared. As a result of using the Exert ability, that creature doesn't untap during your next untap step. One cool thing is that if you find someway for that creature to gain vigilance (so it doesn't tap when attacking) or otherwise untap it, then you essentially pay nothing for the ability.
As Glorybringer is a Game Day promo, this is a card that we'll be seeing a lot of, so is it actually any good? A 4/4 flyer with haste isn't bad for 5 mana. His Exert ability is pretty decent, as well, dealing 4 damage to a target non-Dragon creature an opponent controls. Considering that this is a repeatable ability, it's decent enough.
Is this guy going to see play? It's possible. Will Glorybringer be this Standard format's Stormbreath Dragon? The fact that he can take out a problem creature, too, is definitely relevant. He's definitely worthy of this promotional printing. Glorybringer should be a creature that many Standard players will be looking to slot into the top of their mana curves.