A Protein Skimmer aka Foam Fractionater, removes Dissolved Organic Compounds (DOCs). DOCs stick to the air/water interface and is often seen on unskimmed tanks as an oil slick on the surface of the water. A skimmer will create gazillions of microbubbles and pass them through the tank water in a chamber. The DOCs will stick to the microbubbles and end up as foam that is eventually pushed into the collection chamber of the skimmer.
This device mimics the action of waves crashing over the reef.