If you mean in Climate Control terms, it refers to the Air Source. The Recirculate setting will close off outside air vents and blow the cabin air through the Air Conditioning (Heat or Cool either) over and over. It can help the car heat up or cool down more quickly. It can also block out exterior odors, like that skunk you just ran over, or the refinery you're driving by.
Be careful though to not leave it in recirculate mode. If you discover that your windows are starting to fog up, first check to ensure that the recirc button is not engaged. It can get steamy pretty fast, especially if you have a car full of people.
