Honey bees use propolis to cover and protect their hive. It is a special concoction of resin and bee enzymes, and has been found to be have antibacterial and other properties that help a colony stay healthy and well. Small openings in a hive tend to be filled with the stuff, including the spaces between boxes, covers, and other aspects of the modern hive. This is one reason beekeepers love their hive tool, as it is often the only way to pry apart boxes or covers to gain access to the hive.
Propolis is also used in the winter to reduce the size of an entrance to keep wind and other insects out of the hive.
My top bar hives typically do this every winter, as you can see in the photo. The shiny brown stuff is a thin layer of propolis that the bees built to make their entrance more manageable. The left side is open where bees can come in and out, and there is a bee-sized hole towards the right. In the spring they will tear this down to make it easier for foragers to get in and out of the hive.
It is a curious thing, though I always take comfort when I see it every year, as I know the bees have prepared themselves for the winter.
May you prosper and find honey.