The nitronate has two different resonance structures, one with a negative charge on the α-carbon and a double bond between the nitrogen and one of the oxygens, and another resonance structure with a double bond between the nitrogen and the α carbon, and no double bond between the nitrogen and the oxygens.
The nitronate molecule has two different ways to be represented in its electron distribution, one where the negative charge is on the alpha carbon and there's a double bond between the nitrogen and one of the oxygens, and another where the double bond is between the nitrogen and the alpha carbon but not between the nitrogen and the oxygen.