The Mechanic can repair nearby Vehicles, much like a medic can heal nearby Infantry. When an injured vehicle is nearby, the mechanic will automatically heal it.
A mechanic can revive a killed vehicle by entering its Husk at the cost of vanishing. A revived vehicle will have low health. Note that not all vehicles leave husks, so not all vehicles can be revived by a mechanic.
The mechanic is not to be confused with the Engineer.