Each hydrogen atom makes a single bond. Each carbon atom will make four bonds. The nitrogen atom will make three bonds. In this problem, a double bond must exist between two atoms. A double bond can exist between the two carbon atoms. Or a double bond must be between a carbon atom and the nitrogen atom. As you look at the resonance hybrid structures that pisgahchemist drew, you can that first structure has a double bond between the second carbon atom and the nitrogen atom. For the second carbon atom to have four bonds, the first carbon atom must be bonded to three hydrogen atoms. As you look at the second resonance hybrid, you can see that the first carbon atom is only bonded to hydrogen atoms. But the nitrogen atom is bonded to two hydrogen atoms. During this resonance process, the position of the hydrogen atoms changed. For the number of hydrogen atoms that are bonded to a carbon or nitrogen atom, the oxidation number of each of these elements must change. I guess that we could call this an oxidation – reduction reaction. I hope some of this is helpful for you.