"Medic Droids heal injuries with advanced bacta-based technologies. They are programmed to follow and provide aid to infantrymen closest to them, and are most effective at healing members of the Rebel Alliance."
~In-game description of Medic Droid
Medic Droids heal Rebel infantry units during battle. Every few seconds, a Medic Droid will release a circular wave out of its center and increase the health of units within a five tile radius.
Medic droids tend to be used with Wookiees (including Chewie) and Heavy Soldiers. You shouldn't rely completely on them, as a single Rapid Fire Turret can kill one in a few seconds, which can cause problems when they heal Wookiees and turrets are grouped together