Radio signals are waves of energy measurable by frequency (number of wave oscillations per second).
Because frequency is time-dependent, movement toward or away from a radio wave will affect the frequency at which it is detected. (This phenomenon is known as "the Doppler effect.") One way to control for this movement is to measure barycentric frequency instead of raw frequency. Calculations of barycentric frequency control for the rotation of the Earth and its orbit around the sun.
Also see our information on de-chirping data, a method of controlling for the movement of signal-emitting sources.