Most modern automobile engines use a timing belt or chain to synchronize crankshaft and camshaft rotation. That means All of the spinning things on the top of the engine have to line up with all of the spinning things on the bottom of the engine to a precise tolerance. Those spinning things are connected by a chain or a belt. Failure of the belt is catastrophic to the internal parts of the engine and in most cases damages the engine beyond repair. 

Changing the timing belt is critical for the longevity of the life of the engine.

Timing belt kit; water pump, pulleys, tensioner and belt

Timing Belt vs Timing Chain

...timing belt is a critical maintenance item to ensure the longevity of the car 

 In the last few decades, engine design has brought in the use of overhead camshafts (the above mentioned upper spinny things). This moves them further from the crankshaft (lower spinny thing) and in engines with multiple camshafts, a timing belt or chain also enables the camshafts to be placed further from each other. Timing chains were common in engines produced through the 1970s and 1980s, before timing belts became the norm.