UV or ultra violet sterilizers are a way to remove bacteria, parasites, and many other things inlcuding algae from your aquarium.
Getting the right size of UV sterilizer is important since the amount or GPH of flow through them directly affects the kill ratio. Higher flow rates will kill less and lower flow rates will almost kill everything that passes through them. If you have a problem with algae, ich, or bacteria UV sterilizers can make a huge difference on your aquarium. Keep in mind though that UV will also kill beneficial along with the bad, so we generally recommend running them on FOWLR or FO systems (fish only with live rock or fish only).