Lens cleaners are useful things to have, as a dirty, well dusty more than dirty, lens can cause the machine to skip. They typically are a cd with a small row of soft bristles on the underside of the disc. The disc is played like a normal cd and the bristles brush dust off of the laser lens. When I was using my CDP every day, i'd use my lens cleaner every few months.
Don't pay silly money for one made by a big brand, though, because you're just paying extra for the brand. Have a search about and get one for around a fiver.