What is "dot" sign in •NO?
Usually the dot is put there to emphasize that the nitric oxide is a free radical that includes an unpaired electron.  This is especially notable by comparison with $\ce{NO^+}$, which does not have the unpaired electron.

Note that the nomenclature $\ce{·NO}$ should not be rendered as showing the unpaired electron on nitrogen.  The unpaired electron is actually incorporated into the molecular orbital structure of the nitric oxide molecule, covering both atoms.