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, with one structure featuring a negative charge on the α-carbon and a double bond between the nitrogen and one of the oxygens, and another where the double bond is between the nitrogen and the α carbon but not between the nitrogen and the oxygen.