Many modern vehicles have engine trim which is usually just for aesthetic purposes only, but sometimes protects the engine in some minor way. It is usually plastic material with the engine's size or appealing qualities written on it, like variable valve timing, displacement, manufacturer name, or even an engine model name. They are almost always made out of high temperature plastic, and stylized in some way to give the under-hood appearance more appeal to the average consumer.
Usually this trim is meant to hide the engine, due to a naturally unappealing or complex appearance. In a small number of cases, the trim is actually used to channel air around the engine for cooling purposes. Sometimes, the trim has functional parts integrated into it, such as air intake tubing.
Many enthusiasts remove engine trim, if possible, to give their engine bay a more "real" look, and so it doesn't have to be taken off and reinstalled every time work is done to the engine. The trim parts are sometimes reinstalled for car shows and events, but keeping them off the car until then keeps them clean.