What is Rotation curve; Oort constants?

The rotation curve of a disc galaxy is the speed $V(R)$ of a circular orbit in its plane as a function of the distance $R$ from its centre. For the Galaxy, with $V_0 = V(R_0)$ and $V'_0 = \mathrm{d}V/\mathrm{d}R$ at $R_0$, the Oort constants are