If something is hollow, it is empty on the inside. A chocolate Easter bunny looks like so much chocolate until you realize that it is hollow and quickly eaten.
The idea of emptiness is apparent in the other uses for hollow too. A valley between mountains is called a hollow and if you're really thin, you may have hollow cheeks. Things like words and promises are hollow if the person uttering them does not intend to keep them. And a hollow victory is not very satisfying, as it means that you have reached your goal, but lost something more important along the way.