raise their young and keep their honey stores to eat during the winter. Generally you need about 60 lbs of honey for the winter, but I always add a top feeder of sugar mush in case they run out (I will post a video when I do it in a few weeks).
The smaller boxes on top are called mediums or honey supers. Usually you have to place a device between the deeps and the supers called a "queen excluder" to keep the queen from going up on your honey super and laying eggs. The spaces are large enough for the workers to get through, but generally to small for the queen to get through. One year one of my queens got in the upper boxes and laid a box full of eggs. We had to find the queen, get her below, then we waited for the bees to hatch and they they backfilled them with honey.
The insects never seem to stop fascinating me. I am still feeling so satisfied from extracting the colony from the guy's family room ceiling a week and a half ago.