Poop boards are boards under the roost wide enough to catch poop during the night, about a foot to either side of the roost. Poop boards are usually two to three feet off the ground, with the roosts being six to twelve + inches above it. Poop boards are usually for smaller flocks, since they require you to clean them every morning or every other morning, a chore that grows with the number of poopers.
Poop boards can either have a lip and be filled with sweet PDZ, or be just a board and scraped off. If it is filled with sweet PDZ (granulated PDZ, mind) and sand, the poop is dried out and doesn't stink. It is scooped out with a fiine litter scoop. If the board is just a board, than it can get kinda gross, but the poop is scraped off into a bucket and there is no extra dust. The poop from either board should go into the compost.
The space under the poop board can be used for a variety of purposes, or just as coop space. Some people put nesting boxes under their poop board, or fence it off into an integration safe room. I keep my feeder under the poop board. You could use it as storage or a broody/brooder room too.