Common-mode rejection is the ability of the differential amplifier (which sits between the oscilloscope and probes as a signal-conditioning preamp) to eliminate the common-mode voltage from the output. Now, the ideal differential amplifier would remove all of the common-mode signal, or the voltage common to both sides of the differential pair.
Common-mode voltages can come from numerous sources, including an ambient radiated signal coupled into both lines, an offset from signal common created by the driver circuit, or a ground differential between the two ends of the differential circuit. Regardless of its cause, it’s not the common-mode voltage that’s of interest, but rather the differential voltage. Thus, the measure of how good the differential amplifier is at getting rid of common-mode voltage is its common-mode rejection ratio, or CMRR.
Be mindful that the CMRR of an amplifier changes with frequency. High-performing differential amplifiers start at very high CMRR ratings of as much as 100,000:1 and can maintain high CMRR values over a wide bandwidth. Meanwhile, high-voltage differential probes will provide good CMRR at dc. But as signal frequencies rise, CMRR deteriorates.