The many different components of a vehicle rely on power from the engine to run, and this energy is transferred by means of belts. Older vehicles were built with multiple belts, with each controlling one or two different components, but most modern cars rely on one drive belt to deliver power to the majority of the engine's different parts.
The belt winds its way through the engine, wrapping around an assortment of pulleys attached to things like the water pump, power steering, alternator and electrical equipment. It spins pulleys that in turn transfer the power to the components to which they are attached. A belt must have the proper tension to function, but the nature of the rubber material the belt is made of means that it will lose its elasticity over time. This is where a serpentine belt tensioner comes in.
This part of the belt system automatically holds the belt at a certain tension when the engine is running, as opposed to older systems that had manual tension adjusters. These would require drivers to re-adjust the tension as the belt wore out. If they did not do this, the belt system would likely emit a high-pitched screeching sound when the engine was running.
Modern automatic belt tensioners consist of swing arms, tension springs, pulleys and pulley bearings, and they are built to adapt to the changing state of the serpentine belt so that drivers don't have to concern themselves with the constant readjustment to keep things running smoothly.
However, this is not a permanent solution. The belt can break or come loose at any point, and even if this does not happen, it does have a shelf life. This may vary between vehicles, and the owner's manual will contain guidelines for how often this part should be changed.
If the belt comes off of its track or snaps, none of the components that rely on it for power will function. It may still be driveable, but unless you are close to your destination, it is not recommended to do so. When the belt fails, the water pump will not work, which can cause the engine to overheat and create more problems than just a broken belt. The battery will also lose the ability to recharge without a functioning drive belt.