Benzene is a resonance hybrid mainly of two Kekule structures. Hence :

Each carbon atom in benzene is connected to two other carbon atoms in Kekule's structure, which arranges the carbon atoms in a hexagonal ring.
There are alternate single and double bonds in the ring.
The electrons in benzene that cause these double bonds are present.

These electrons have the ability to delocalize and create two distinct double bond positions in their structures.

These contribute to resonance. In order to create a resonance hybrid, they both contribute equally.