A follower board (aka dummy board) is the same shape and size as a normal frame, but instead of having foundation or comb in the middle, it is solid wood. They are usually thinner than a normal frame too. They can be used to fill empty spaces, for example if you you have a lot of extra space with only 8 frames in a Flow brood box. Here is one from Mann Lake:
They can also be used to divide a box into smaller sections. In the Hogg cassette box, they are placed on both outer sides of the cassettes with springs behind them, to provide pressure from the hive walls, holding the plastic cassettes firmly in place. Any clearer? Following me now??