A valve job is an operation which is performed on any four stroke cycle, internal combustion engine, the purpose of which is to clean up the mating surfaces of the poppet valves that control the intake and exhaust of the air/fuel mixture that powers the motion of the pistons.
In the earliest automotive engines, the valves needed to be removed and the sealing surfaces sanded, ground or lapped multiple times during the life with hardened valve seats that resist wear, , internal combustion engine, of a typical engine.
As the decades passed, however, engines ran cleaner and the addition of tetra-ethyl lead in gasoline meant that such maintenance became less frequent. Today, valve jobs are rarely done on passenger cars for the purpose of maintenance, although they are still quite common with high-performance cars.
Some older cars have relatively soft valve seats that wear faster when operated without leaded fuel. These usually can be updated with hardened valve seats that resist wear, regardless of whether leaded fuel is used.