Empty Avatars refers to certain "players" that join just as the game starts. These "players" are actually controlled by another, actual player in the game, the manner in which this control happens is different for each role.
Puppets are controlled by the Puppetmaster. They start with 2 Puppets and can freely talk out of either Puppet or their main avatar at any point during the day phase. While Puppets can vote, they are worthless so cannot cause a lynch to happen.
The Zombie is controlled by the Voodoo Doctor. They start with 1 Zombie and can freely talk out of either the Zombie or their main avatar at any point during the day phase. While the Zombie can vote, it is worthless so cannot cause a lynch to happen.
When the Voodoo Doctor chooses to kill, the target and their Zombie switch avatars before killing the target. Therefore, it appears as though the old Zombie has died, and their victim continues to live on.
Corpses are controlled by the Revenant. They start with 3 Corpses in the Graveyard. So long as the Corpse is alive, they can freely talk out of either a Corpse or their main avatar at any point during the day phase. Corpses can vote and their votes do have value.
During the night, the Revenant can choose to revive a Corpse. To do so, they need to target another non-Corpse body in the graveyard first. Then, the Corpse and the target switch avatars before the Corpse is revived. Therefore, it appears as though a player has been revived and not a Corpse.
The Monster is controlled by Frankenstein. It starts in the Graveyard and appears as a Corpse to other roles. Nothing is else known about it's capabilities at this time.