A vehicle's grille is almost always an opening in the front designed to let enough air in to pass by the fins of the Radiator, which is necessary to cool the engine. Some vehicles only use vents (often the case in rear-engine and mid-engine vehicles) for this purpose, and air-cooled engines don't have a radiator, but still use air from the grille for cooling purposes.
The position of the grille in relation to the front bumper can affect how much air gets through it, as well as how much mesh material it consists of and its angle.
Early vehicles had radiators placed right up front so that the radiator itself formed part of the body shape. The air still had to pass through the radiator and end up in the engine bay, but the body wasn't used to cut down on how much air entered the radiator.
The 1932 Ford Model B Coupe pictured at the right has a radiator style grille, where the front has a radiator shape but isn't actually a radiator. Note the color seen through the lower portion. This was a styling choice during the transition from radiator-front designs to the grille design of modern vehicles.
Grille Function In Motorsports
The grille is a necessary disadvantage in many forms of racing; although it is allows for air to come through the radiator for cooling, the radiator itself generates a lot of drag and after passing through it, the air has to find its way through the engine bay.