It is the velocity with which a star, in this case, moves away or approaches us and is perpendicular to the transverse velocity that would correspond to its displacement across the sky. And it is radial because its direction draws radii from the center of mass of the star's rotation, which is not necessarily the center of the star, since it is displaced by the gravitational pull of the planet or planets orbiting it.

This velocity is measured by the Doppler effect which is reflected in the spectrum of the star as a shift of its spectral lines to one side or the other of the spectrum.