The Hori DS Lite screen protectors are a set of clear plastic sheets meant to protect your DS from scratches and dirt. The kit includes 5 sheets: one for each screen, two to cover the button areas on the inside, and one for the top cover of the system.
I've found that one they're on, these protectors work quite well. They make it easy to get dust off, and free you from anxiety about damage to your DS. However, I had some serious problems getting them on the DS. The protectors are very thin and adhesive, and it's almost impossible to get them on without trapping some dust or air bubbles underneath the protector. It's also sometimes difficult to separate the protectors from the sheets they come covered with. If you can get past this initial difficulty, screen protectors are a good investment, but make sure you can get them on properly.