To be as unscientific and as clear as possible, let's simply say that the air bubbles inside the skimmer's body strip the water of undesirable waste by-products. How the bubbles accomplish this is a neat trick that needs some explanation. Ever blow bubbles as a kid? Remember all the rainbow colors on them? Just as the soap clung to the giant bubbles you were creating so too, does all the junk and other organic gunk in your aquarium water. Those pretty rainbow colors were the light refracting off the soap film...you could actually see it! In our skimmers, the bubbles are microscopic and the results can only be "seen" after they burst and deposit their "films" into the collection cup! No pretty rainbow of color here...nope. Only the vilest and nastiest looking sludge imaginable ride our skimmer's bubbles.
How does this happen? It was discovered long ago in waste treatment plants that by injecting high volumes of air bubbles into a column of waste water, the resulting effluent was purer and much cleaner than before. How could this be? Actually, quite simple. Surface tension. Surface tension? What's that? The interaction between the oxygen bubble and the surrounding water creates a kind of friction between the two. This friction in turn "charges" the molecules in the water.