An IPM Board, or Integrated Pest Management Board, is used to help you determine the amount of mites in your colony. It is composed of a screen bottom with rails mounted underneath so you can slide in a sticky board (included.) Cover the sticky board with something like Pam or Vaseline so the mites stick to it. Sticky boards are made of corrugated plastic and can be washed and re-used.
You can also use the rails to slide in plywood during the winter to close up your hive, without having to switch bottom boards.