What is the "resonance hybrid" of benzene? In my book, to explain the structure of benzene, it referred to the idea of resonance:


  The idea of resonance is that the actual molecule is a definite structure that is a hybrid of two or more theoretical structures.


I partially understood this sentence, specifically how a molecule could have multiple structures, but am confused about what is meant by hybrid. Could someone please elaborate?
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.