The radiator shroud prevents the recirculation of air around the fan. It is usually a plastic hood that encloses the fan to guide the air through the core, and stop it from coming back around and through the fan again. It also protects you from the fan blades. The plastic covering does tend to break or come loose, so it is worth checking and making sure you are careful when you are working on the engine. If the fan that pulls air through the radiator core to cool the engine coolant is too far back, it will end up recirculating the same hot air that has collected behind the radiator. For this reason, the radiator often has a shroud.