Your car's engine has a camshaft or a few camshafts and each one needs to turn to allow the intake and exhaust valves to open and close to allow air and exhaust gases to come in and out of the engine. Some cars use a belt or gears but most cars have a timing chain to accomplish this task. This part is absolutely vital to the operation of a car or truck internal combustion engine and should be carefully checked and properly lubricated to avoid damage. It is very hard to predict when a chain will break but you can be sure it will happen as far away from your driveway or your mechanic as possible. The engine will not be able to run at all if the timing chain breaks, but that may but the least of your worries. If it breaks at speed, it can cause a lot of damage to many other parts inside the engine. The intake and exhaust valves, the camshafts, the pistons can all be damaged when a timing chain goes. Replacing these parts is an expensive and time consuming job. Refer to your owner's manual or shop manual for expected intervals to replace this chain.
In the early days of cars, they used gears running directly off the crankshaft to run the camshaft. The timing chain allowed more space between the crankshaft and camshafts. The chain is made out of interlocked pieces or "teeth" of metal to allow it to bend around the round pulleys of the engine. In the 1970s, a timing belt came along made out of rubber. The belts tend to be lighter, quieter and less expensive than the chain. In the last decade or so, more cars have switched back to a timing chain. The chains tend to last much longer than the belts and the sound difference is virtually nil. Some cars may have both a belt and chain or may even still have a gear.
Replacing a timing chain is a very important job that should only be done by experienced mechanics. It is a complex job that requires a large variety of tools but the basic instructions are shown below.