Some engines are designed to be cooled by air. These engines don't use a radiator or coolant, and instead get cooled down by air passing over its surface.
Because it doesn't use a radiator or liquid coolant, it has much less to carry around, making it lighter, and this also means there are less parts to break and corrode.
These engines require a constant flow of air to keep cooled down, so it's easy to overheat when not moving anywhere or if there isn't enough air flow in the engine bay. The need for extra air flow also may increase the level of aerodynamic drag.