What does "A Target with Zero Doppler" Mean?
In Pulse Doppler Radar, the Doppler frequency that a target's return signal exhibits can be used to calculate the Radial Velocity of target.  That means, the return frequency of a pulse will be different than the frequency of the pulse that was sent out depending on how fast the target is moving toward the source of the pulse.  If the target's "Radial Velocity" is zero, zero doppler will result, i.e. the resulting doppler caused by the target's Radial Velocity will be 0 Hz.  

Wikipedia shows the calculation of Doppler to be:

$$
F_{Doppler} = \frac{2 * F_{Transmit} * V_{Range} }{c} 
$$

$c$ being the Speed of light.

So, if the target's Radial Velocity is zero (not moving towards or away from the observer), $F_{Doppler}$ will also be zero.

https://en.wikipedia.org/wiki/Pulse-Doppler_radar