Propolis, or bee glue, is the resinous substance that the bees gather from tree buds and it has a multitude of purposes inside the hive. Not only do bees use propolis to seal the hive and protect it from the elements, but they also use it to sterilize the cells that will house their eggs. The bees learned long ago that propolis protects the hive from mold, mildew and microbial contamination.