A revive allows a soldier whose health has hit 0 to be brought back into combat without the need of being respawned. This allows for a soldier to get back into the combat more quickly, and it also allows a soldier to avoid having his respawn time lengethening due to frequent deaths. However, a soldier who is revived has the same stamina and armor as when they died, so often, a soldier's armor needs to be repaired and the soldier needs to rest for awhile to regain their stamina.
To revive a fallen comrade, a player needs the Advanced Medical certification along with a Medical Applicator. Reviving someone takes 25 units of ammo from a Health Canister and takes several seconds. During this period, the Medic is vulnerable to attack, so other players should attempt to cover a medic who is trying to revive someone.
When trying to revive a player, the Advanced Medic needs to be close enough so that the "Corpse of" dialogue comes up underneath the reticle. Also, when pointing the Medical Applicator at the ground, the "Corpse of" dialogue needs to show up to be able to revive the player. Generally, the area around the feet is the easiest area to find the "Corpse of" dialogue.