CMRR is a measure of the capability of an op-amp to reject a signal that is common to both inputs. Ideally, CMRR is infinite: if both inputs fluctuate by the same amount (while remaining constant relative to each other), this change will have no bearing on the output.
Common mode rejection is a feature of differential amplifiers. Op-amps are amplifiers with differential input; so common mode rejection applies to operational amplifiers. Common mode signal is when both of the inputs of the amplifier have the same voltage or they have a “common voltage” across them. Under this condition the output of the amplifier should be zero or the amplifier should reject the signal and not amplify it.
The picture below shows the MCP601 common mode rejection specification.