Medics are units that are housed within the Medic Station or Bunker. Medics will automatically retrieve battlefield casualties and return them to the Medic Station/Bunker for medical treatment and redeployment. They are completely autonomous and never directly controlled.
Medics can help you get free squads at no cost, which helps your war effort with minimal cost to your economy.
Medics are spawned for free from medic stations. Unit AI will not target medics by default, though a player can manually order units to shoot at enemy medics. Likewise, computer players in skirmish mode will ignore medics belonging to an enemy team. Although firing on medics is considered a war crime in real life, there is no negative consequence for doing so in-game, and in fact it is beneficial to do so to prevent your opponent from building up free infantry squads.