There are many different ways to make Sponge filters at home. Alot of youtube videos exist under "DIY Aquarium Sponge filter."
They are one of the most "old school" methods of filtration that became extremely popular in the 1980s, built into very simple aquariums, because they are so simple.
A basic DIY Sponge filter involved an air pump, an air line, an air stone (somewhat optional), a plastic cylinder shorter in length than the lowest and highest points of your water line in your aquarium, and one of the following: A sponge, a Cotton-like mesh, a filter sock, panty hose and gravel - at the top of the plastic, obstructing the bubbles from traveling straight through to the top of the cylinder.
There are various ways to use it, but as simple as it is, and as simple as Mister Spark pointed out, the airline tubing w/ or without air stone reaches down behind the tube. The tube lays just tall enough above your substrate to allow water to reach into the tube. By a matter of inverted water based gravity (Air pressure), the bubbling air from the air stone causes water to pass with it up the tube and out through the filter media up top.
This builds together a fantastic amount of beneficial bacteria in the end of the tube, and as said before, it is a very simple, mildly disturbing to your air movement filter medium. This is most common in fry and hospital tanks.