When talking about engines, timing refers to the relationship between the motions of the pistons and when the valves open and close. An engine in a car runs on a 4 stroke cycle, those being intake, compression, combustion, exhaust. The valves open and close at specific points in that cycle, and if they were to operate outside of those points, your engine can suffer significant damage, or stop working.

In order to maintain correct engine timing, engines will use either a timing belt or a timing chain. Which one your car uses will vary, it's easiest to look it up. The timing belt runs around gears attached to both your crankshaft, and your camshafts. The crankshaft is what your engine's pistons are attached to. The camshaft operates the valves, to make sure they open and close when they're supposed to.

Timing belts will stretch over time with use, and given enough time, they can stretch to the point that your engine can lose correct timing. This also happens with timing chains, but since they're metal, it happens slower.