In a radar system, a transmitter sends out a signal, often pulsed in nature. At the same time, a receiver (usually colocated with the transmitter) looks for echoes of that transmitted signal that occur due to reflections off of the radar's targets. These reflections will vary in time delay, amplitude, and frequency from those that were transmitted.
It's hard to get any information from the received amplitude because so many factors can affect it, but the others are important:
Time delay (the measured time between the transmitted signal and the reception of its echo) corresponds to how far away the target is. If you know the time delay and the signal's speed of propagation, you can calculate the distance to the target that caused the reflection. In radar parlance, this quantity is often called range.
Frequency offset (the difference in apparent carrier frequency between the transmitted signal and its echo) corresponds to the radial velocity between the target and the radar, due to the Doppler effect. Based on the observed change in frequency, one can calculate how fast the target is moving as a fraction of the signal's speed of propagatation. Note that only the velocity component in the vector direction between the radar and target can be measured in this way. Radar systems usually refer to this quantities as range rate.