An undergravel filter is simply a slotted piece of plastic (undergravel plate) that sits on the bottom of the aquarium and lets water flow between the plate and the bottom of the tank. There is a tube that is attached to this bottom piece, known as the lift tube. Gravel is placed on top of the filter, and the filter, lift tube and gravel work together to provide filtration.  Water is pulled down to the bottom of the tank, through the gravel. And then the undergravel plate, then flows back up to the top through the lift tube. The water is again pulled down through the gravel. This allows beneficial bacteria to thrive on the oxygen laden water flowing through the gravel.