Whenever a single Lewis structure cannot describe a molecule accurately, a number of structures with similar energy, positions of nuclei, bonding and non-bonding pairs of electrons, known as the canonical forms of the hybrid are taken, which describe the molecule accurately.
This phenomenon is known as resonance.
Resonance hybrid structures of carbonate ion are shown as follows :

(i) The relative position of all the atoms in each of the canonical forms is same.
(ii) The number of unpaired and paired electrons in each of the canonical forms is same.
(iii) Negative charge is residing on more electronegative atom.
(iv) As a result of resonance, bond length and bond order is same.