By holding the Repair button, Darius Mason emits a stream of blue nanites that reverse damage to most objects in the environment, within a few meters. Later, he also becomes able to eject "repair grenades", which can repair at any distance, by tapping the repair button.
The Repair function allows the player to re-create bridges and other necessities of the environment, often serving as a plot element. It also plays an important role in the Defend maps of Infestation, though only repair grenades are permitted.
Repair appears to be intended to only work on non-biological material. Buildings can be restored from rubble, but not buildings infested by the Plague. Dead bodies and other biological material, once destroyed, cannot be replaced. Explosive barrels and tanks cannot be replaced.
Violations of these guidelines can be found occasionally, like reparable corpses, but these can be assumed to be developer oversight.