Complex sentence: Displacement of these peaks away from their known frequencies is called "echo Doppler shift" and permits to obtain the radial velocity of a surface current, that is the scatter velocity along the line between the hit surface and the radar.
Split into simpler sentences: Displacement of these peaks away from their known frequencies is called the "echo Doppler shift" and allows one to assess the radial velocity of a surface current. That is; the scatter velocity along the line between the hit surface and the radar.