If I am holding a cup of water and I ask you, "Is the cup empty?" you will say "No, it is full of water." But if I pour out the water and I ask you again, you may say, "Yes, it is empty." But, empty of what? My cup is empty of water, but it is not empty of air. To be empty is to be empty of something.
When Avalokita says that the five skandas are equally empty, to help him be precise we must ask, "Mr. Avalokita, empty of what?"
The five Skandas, which may be translated into English as the five heaps, or five aggregates, are the five elements that comprise the human being. In fact, these are really five rivers flowing together in us: the river of form, which means our body, the river of mental formations, the river of feelings, the river of perceptions, and the river of consciousness. They are always flowing within us.
Avalokita looked deeply into the five skandas and discovered none of them can be by itself alone. Form is empty of a separate self, but it is full of everything else in the cosmos.
The same is true with feelings, perceptions, mental formations, and consciousness.