Inside each box in a beehive are frames the bees build their comb on, forming cells to store their honey, pollen, and to raise their bees in. Fortunately, the frames are made to be removable so you can easily inspect the health of the colony and harvest honey. The ability to remove the frames easily is what makes Langstroth beekeeping equipment so popular. The frames and comb on them is reusable by the bees and beekeeper and can actually be used for decades with good care
Most frames are made from four pieces of wood and have a space for a sheet of foundation to be placed into the frame. The foundation is embossed with small hexagons that act as a blueprint for the bees to build their comb on. Honestly, they don't need help building comb, but it does make it a little easier for the beekeeper. Each frame is designed to work in a specific box, so you aren't able to swap any frame for any box. Deep frames, foundation, and boxes are made to work together. Medium frames, foundation, and boxes are designed to work together.