Resonance theory explains various observed properties in a molecule using the electron delocalization concept and multiple Lewis structures, which a single Lewis structure cannot.
A single Lewis structure can only describe some but not all of a molecule's observed properties. Resonance theory is helpful in molecules that can be expressed using several Lewis structures, like Benzene or CO2.
The several structures that show delocalization of electrons, mainly pie electrons over a conjugated system, are called the contributing or resonance structures, each separated by a double head arrow. The molecules that require such multiple resonance structures are said to show resonance.
For example, resonance structures 2 and 3 explain the triple and single bond nature of CO2, whereas structure 1 describes its double bond nature.
In drawing resonance forms, the structures are drawn that are as low in energy as possible. The best contenders have the maximum number of octets, maximum number of bonds, negative charges on electronegative atoms, and as little charge separation as possible.
A resonance hybrid is an average of all resonance structures and is closest to the real nature of the molecule.
Read more about Resonance.