A fender liner, also known as an inner fender liner, is a plastic barrier that separates the fender from the engine in most modern cars. Each of the front wheels has its own fender liner. Its job is to keep dirt, mud, water, environmental toxins, slush (in snowy areas), and other impurities out of the engine bay, where they can cause serious corrosion and damage. It also helps to prevent corrosion of the wiring and other components inside the wheel well.
If you live in locations prone to an environment that will cause extensive damage to your fender liners, then it is important to have them maintained or repaired!