Although follower boards (also known as dummy boards) are often used in top-bar hives, they are less often seen in Langstroth-style equipment. But many who use them are firm believers in their ability to lessen swarming in the summer and insulate the hive in winter.
Follower boards in a Langstroth hive have the same shape as a regular frame except the top part of the frame is only about ¾” wide. Instead of filling them with foundation you fill them with masonite or some other thin, solid material like plastic or ¼” plywood.
You simply take one regular frame from the brood box and replace it with two follower boards, one on each side of the brood, in positions 1 and 10. That is why the top part has to be narrow—because you replace one frame with two followers.
The theory here is that the bees can collect on the follower boards without sitting on the brood. In hot weather, the bees have a hard time keeping the brood cool enough, and sitting on it makes it worse. So both follower boards and slatted racks give the bees a place to “hang out.” This also reduces the feeling of congestion in the hive and congestion is a major factor in swarming.
There should be “bee space” on both sides of the follower boards in order to provide lots of room for the bees to go. In winter the bees will not use the boards for clustering, but the dead air space between the boards and the outside wall provides some insulation against the cold.
Some beekeepers like them because a hive with nine frames and two followers is lighter than one with ten frames full of brood, honey, and pollen. Of course that depends on what you use to build them. Other people think it makes no difference in the total weight.
Beekeepers who are not fond of the geometry of the Langstroth hive say the use of follower boards creates a more optimum space for the brood nest. I have no clue about that myself, but I plan to try some experiments in the near future.