Protein skimmers are used in marine aquaria to remove organic waste, mainly. It acts as a filter physically removing solid wastes.
A protein skimmer - water flows into a chamber and comes in contact with a column of fine bubbles. ( the smaller the bubbles the more effective because in the same volume they will have a bigger surface area)
Then the bubbles will collect the proteins and other substances ( such as bacteria,metals like copper, fats and fatty acids) and carry them to the top of the device where the foam (which is created by the small bubbles) collects in a cup. These cups keep the accumulated junk from going back into your aquarium) The foam condenses to a liquid, which can then be easily removed from the system.
This will improve the quality of the water in your marine setup.
It isn't complicated as it sounds but one has to read about it and find the best solution for his aquarium.