Busywork is work you are given so that you look busy, or so that the time is filled, but it does not really need to be done. A teacher might hand out busywork when the real lesson is over. An office might make people fill in forms that nobody will read. The point is the busyness, not the result. It is mainly American (often written busy work as two words). It always sounds critical: the speaker thinks the work is a waste. It is not a compliment, and it is not the same as hard work that has a real purpose.

無謂的工作、填時間的工作（不是「很忙的工作」；偏美式）