A baffle in a motorcycle exhaust pipe is a sort of muffler designed to reduce engine noise. There are two kinds of baffles; the type placed inside the exhaust pipe works by reducing air flow out of the exhaust.
The reduced air flow from the baffle creates a differential in air pressure that forces some of the exhaust back into the engine. This not only reduces sound, but also helps the engine perform better. Unburned fuel gets forced back into the engine along with the exhaust and is utilized, improving both fuel consumption and horsepower.
A baffle silencer, which is another type of baffle, is placed at the end of the exhaust pipe. Using a resonating chamber, it creates sound waves that cancel out the waves generated by the engine.