A splitter is a small wing attached typically to the front of a vehicle in order to provide downforce at high speeds. They may be molded into or permanently fixed to the front fascia or fender of the vehicle, or you may find adjustable splitters affixed to the front of the vehicle below the front fascia which can be adjusted up or down via a turnbuckle to control their level of provided downforce, and thus drag.
Splitters are usually made of plastic, fiberglass, FRP (fiber-reinforced plastic) or carbon fiber and are typically found on race cars, sports cars, and supercars, or added to sports cars, typically for their visual effect as no significant amount of downforce is produced at speeds less than about a hundred miles per hour.