In such a radio receiver, a local oscillator is used to heterodyne or "beat" against the incoming signal frequency, generating sum and difference frequencies - one of these will be at the intermediate frequency or IF, and will be selected and amplified. The radio system is responsive to any signal at the IF frequency, and it is possible for an unwanted signal at the input to cause this - for example if the wanted signal is 100.0 MHz, and the IF is 10.7 MHz, the local oscillator may be tuned to 110.7 MHz, creating the sum (210.7 MHz) and difference (10.7 MHz). However a signal at the input at a frequency of 121.4 MHz will create sum (232.1 MHz) and difference (10.7 MHz) frequencies, which latter signal will be selected and amplified by the IF stages of the radio receiver. The signal at 121.4 MHz is called the image of the wanted signal at 100.0 MHz. The ability of the receiver to reject this image gives the IMRR of the system.
IMRR is measured in dB, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver. In a good design, ratios of >60 dB are achieveable. Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity) - the signal yields a perfectly valid IF frequency. Rather, it is the measure of the bandpass characteristics of the stages preceding the IF amplifier, which will consist of RF bandpass filters and usually an RF amplifier stage or two.