A protein skimmer is perhaps the most important piece of hardware for the salt water tank. It is a filter used to remove organic impurities from the water. Water is sent through a fractionating column where many tiny air bubbles are pushed through it. The air bubbles generate "foam", which actually consists of organic impurities. The "foam" is channeled out of the device and is kept in a collection cup until it can be discarded. There are obviously expensive and cheap skimmers. Going cheap with your skimmer may be the  worst decision you make in your aquarium hobby. Of course there are very expensive skimmers that while excellent are out of the range of the average hobbyist. There are mid-range skimmers that are great and made with good acrylic and pumps and will do a great job.