Your engine has a lot of moving parts inside. In order for all of those parts to move in concert with one another, they need to be synchronized. Like a conductor directing the instruments in an orchestra, the timing belt (or timing chain in some engines) synchronizes the movement of the crankshaft, camshaft(s), valves and more to make sure that each part is where it belongs at any given moment when your engine is running.
The timing components - timing belt or chain, pulleys or sprockets, tensioners, and the like - are located at the front of an engine. The timing cover essentially serves as the front panel of the engine and protects those timing components from dirt and debris. In the case of an engine with a timing chain (rather than a belt), the cover also seals in the engine oil that keeps the timing chain lubricated.
The procedure to replace a timing cover can differ from one vehicle to the next for a number of reasons, including whether the engine features a timing belt or a timing chain, or whether the engine is front-facing or transverse (as is the case with most front-wheel-drive vehicles). The service is also fairly involved, since a significant amount of labor is required to remove all of the necessary components for access.
In general, a technician will need to lift and support your vehicle in the air, remove shields, guards, covers, ducts, and the serpentine drive belt. The radiator might also need to be removed, depending on the orientation of your engine.
With those items out of the way, the technician will: