Reviving allows a player to get another player out of the hospital, no matter how long they are in the hospital for. Reviving is commonly used by players who are in the hospital for long periods of time, e.g. from overdosing on certain Drugs.
Reviving costs 75 Energy. This means that a donator with a full energy bar could do two revives, while a non-donator could revive one player, but would have to wait for 50 more energy to perform another.