Vents are holes in the shell of the helmet. They have two functions – to reduce the weight of the helmet, and also to add ventilation. Helmets with fewer or no vents are usually considerably warmer, though the latest crop of aero lids has shown the value of well designed internal channelling. This might not be obvious when you try one on in a shop, but once you start working up a sweat climbing a big hill when hot weather cycling, it becomes invaluable.