Resonance is a concept that describes the bonding in molecules…
a. where there is more than one choice of location for a double bond as deduced from the Lewis dot structure. The true bonding is the average over all possible multiple bond locations.
b. by asserting that electrons in a double bond can delocalize (spill over) onto adjacent single bonds to make a bond and a half.
c. by asserting that double or triple bonds “flip” or resonate between two locations in the molecule.