The engine valves (or intake valves) control the fuel:air mixture that goes into the cylinders. The valves open and close when the egg-shaped camshaft lobes press against the valve lifter as the engine turns.
Valves, springs and lifters
When a mechanic refers to the valves, he's likely talking about the valves, their springs and their lifters. Most mechanics consider them to be all one unit, despite being separate components.
Common Symptoms of a Bad Valve
When a car backfires (makes a "popping" or "firecracker" sound) and the problem is not in the exhaust system, then it could be in the valve train. An engine that won't start, is sluggish, or seems to "miss" at regular intervals could also be a sign of bad valves. Finally, a poorly-running engine which produces low performance or "tractors" (is jerky in operation) may have bad valves as well.
Engine Valve Repair or Replacement
Most of the time, valves should be replaced rather than doing an engine valve repair. Sometimes, the issue is just a worn valve spring, other times it might be carbon deposits that can be scrubbed away. Most of the time, it will be a combination of problems with the valve.
Replacing valves and related components is a time-consuming, costly endeavor and one that only the most experienced DIY mechanic should attempt. The tolerances for valves are very tight and the components that must be removed and/or replaced in order to get to the valves are many and complex.