An air-cooled hood has one 4" or 6" air duct input and one for the output. You install an inline fan to bring air in from outside the grow area through the light and then back out the room taking the heat with it. This way you can get the light closer without burning your plants or getting the temperature too hot and causing them to hermie.

A Cool Tube is the same principle but has a round glass tube that you attach the ducting to and can get much closer.