It means that what we usually consider to be the 'structure' of benzene is not the actual structure. See below
While we say that there are three distinct pi bonds in the structure there is actually resonance, or electron delocalization. This means that the electrons are not always in the double bonds, but distributed throughout the ring. This is one of the most important characteristics of an aromatic system.
In the above image the lower structure is the "hybrid of two theoretical structures" - notice the inscribed circle. It represents that those 6 electrons are shared between all of the Carbon atoms. The two structures above that are the "two theoretical structures" that are in resonance with one another