Mechanics:
Embalm
Embalm is an activated ability you can activate if the creature card with embalm is in your graveyard. Notably you're not casting the card from the graveyard, so things that counter spells won't work against the embalm ability. The original creature spell can be countered, of course, but that just puts the card in the graveyard. I think you see where this is headed.
To activate an embalm ability, simply pay the embalm cost and exile the card. You do this any time you could cast a sorcery, meaning during your main phase when nothing else is happening. When the ability resolves, you create a token copy of the card, but as you'd expect, going through the process of mummification causes some changes. It's got that whole undead thing going on, so it's a Zombie in addition to whatever creature types it used to have. You'll find lots of ways to take advantage of its new Zombiehood, I'm sure. And now that it's all service, all the time and doesn't care about whatever it was it cared about while alive, it's white instead of whatever color it used to be. One more minor change is that it doesn't have the mana cost the card has.
Exert
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. You are clever!