The idiom "busy as a bee" is used to describe someone who is very active, hardworking, and productive. Bees are known for their constant buzzing and tireless work ethic as they tirelessly collect nectar from flowers and produce honey.
Thus, the idiom "busy as a bee" implies that the person being referred to is always working hard and efficiently, often to the point of being constantly occupied with tasks and responsibilities. It's a positive and complimentary way of describing someone who is very productive and diligent.