The Vitals ability can be used by all players to check other players' vital signs - if they are alive, dead or has disconnected.
There are a lot of strategies involving vitals, e.g. if someone is killed and the body is immediately reported, it is likely to be a self-report by an Impostor.
This ability can be pretty powerful especially if paired with the Admin Map to deduce information about the kill and the location of the body.
OK means the player is alive.
DEAD means the player has been killed in this round, their body will be somewhere on the map. However, if someone decides to leave the game right after being killed, they will display as D/C instead.
D/C means the player has disconnected.
The players appear in the order that they joined the game.
Wiki for more strategy tips.