The ability to heal is available to the start of Spec Ops: The Line, and you can heal a teammate if they take a large amount of enemy fire and become downed. When Sgt. Lugo or Lt. Adams become downed after taking substantial fire, you either have to run over to them and heal them yourself, which takes around 5 seconds, or use the 'Command' option to send another able player (either Lugo, Adams, or in Chapter 10: Riggs, Riggs can help as well) to heal the player. However, if you run over to heal the downed player and there are too many hostiles firing at you, you run the risk of being wounded or killed, and if you send another soldier over, they may also come under fire and become downed as well, which would give you no choice but to rush over and heal the downed players. The bar will slowly return to normal, provided no extra damage is taken.
When Adams or Lugo are downed, a bar will show and steadily turn red. This is to signal to the player that their squadmate is bleeding out. If you do not get over there before the bar turns fully red, the mission will fail and a message will show saing, "Your Squadmate Died!". If the downed player is shot whilst downed, then the bar will fill red faster, as they would be bleeding out more.
In the Multiplayer section, healing can be used in exactly the same way as in the Campaign, except in Multiplayer you can call for help or choose to "take one for the team" and leave yourself to bleed out. However, if you call a squadmate over, you run the risk of getting them killed, or before the squadmate reaches you, the shooter or the shooter's teammate can find you and execute you.