Exert is an ability that you can choose to use when declaring attackers. 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.
Glorybringer is one of the better Game Day promo cards that Wizards has chosen. 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.
Glorybringer would be the Stormbreath Dragon for each Standard format he appeared in for the eighteen months he was around. Being a 4/4 creature with haste and flying, plus being able to take out a problem creature made him relevant at the top-end of pretty much every aggressive Red deck. He was well worthy of this promo printing. Not only was Glorybringer a creature that many Standard players slotted into the top of their mana curves, but he's even crossed into Modern, too! He is going to be a fun Dragon to play with for years to come.