In general, wireless receivers have a local oscillator that mixes with the incoming RF signal to produce a lower frequency signal at the Intermediate Frequency (IF) that is then processed in the rest of the receiver. For instance, in a 180 MHz, CR187 receiver, the IF is at 21.4 MHz. This is much easier to filter and amplify than the transmitter's 180 MHz carrier. To produce this signal, the incoming 180 MHz is mixed with a signal at 158.6 MHz to produce a difference signal at 21.4 MHz.
The mixer can also mix a signal at 137.2 MHz to 21.4 MHz since the difference between the local oscillator at 158.6 and 137.2 MHz is also 21.4 MHz. Therefore there are two signals that can easily produce 21.4 MHz by mixing with 158.6 MHz: the desired 180 MHz and the "image" of 137.2 MHz. The mixer is equally sensitive to either signal and without a front end, a receiver is just as sensitive at the image frequency as it is at the desired frequency. The RF front end, which is tuned to pass 180 MHz but stop the image of 137.2 MHz prevents any response to the image. This is why the 187 series receivers have multiple helical resonators in the front end. The front end has to smack the image down 100 dB or more. This image frequency should be taken into consideration when doing frequency co-ordination, though with modern receivers, the image is almost completely rejected.