It means that what we usually consider to be the 'structure' of benzene is not the actual structure.
Although there are three π bonds in the structure of benzene, there is actually resonance, or electron delocalization. This means that the electrons are not localised in π bonds between two specific carbons, but distributed throughout the ring. This is one of the most important characteristics of an aromatic system.
The "localised" structures 1a and 1b above are resonance forms of benzene: neither 1a nor 1b are a true representation of benzene on their own; they are hypothetical, or "theoretical", structures.
The resonance hybrid is a weighted average of the resonance forms 1a and 1b. In this case, both forms contribute equally to the hybrid. So, each C–C bond has a bond order of (1+2)/2 = 1.5, which we can represent using dotted lines in 2 (each dotted line represents a fraction of a bond, so one solid and one dotted line means a bond order between 1 and 2).
More symbolically, we can also use the depiction with an inscribed circle 3. This means that the six π-electrons are shared between all six of the carbon atoms.