A Repairman is a unit that is build in the barracks. It has no attack power and barely any HP. Its only use is the repair of buildings and structures. It does not heal any units, only buildings.
Build this unit in the Barracks and move it to the nearest building that needs repair. It will automatically start repairing until the building is at full HP.
This unit seems not to be able to be destroyed by explosive tanks only normal troops and tanks seem to target him