For the reaction, $2{\text{ NO}}_{\left(\text{g}\right)}\to {\text{N}}_{\text{2}\left(\text{g}\right)}+{\text{O}}_{\text{2}\left(\text{g}\right)}$  the expression $-\frac{1}{2}\frac{\text{d}\left[\text{NO}\right]}{\text{dt}}$  represents

The ratio of concentration change to time during very small time period.  So that $-\frac{1}{2}\frac{\text{d}\left[\text{NO}\right]}{\text{dt}}$ represents the instantaneous rate of the reaction.