The Lewis Structure of the carbonate ion, which is CO3(-2), has one carbon atom in the centre and three oxygens around it. In order to complete the octet on carbon, ONE of the oxygens needs to be drawn with a double bond to it - the others are drawn as single bonds. There are actually THREE valid Lewis Structures for this molecule, one with *each* of the oxygen atoms, in turn, being double-bonded the carbon.
The true structure of the carbonate ion is a MIXTURE of all three of those valid Lewis Structures. This mixed structure is called a Resonance Hybrid and I draw it for you in this video, along with the valid Lewis Structures along the way.

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