Why A preselector is a tunable or band-switched bandpass filter placed ahead of the mixer (and often ahead of the RF amplifier) so that only energy in or near the desired band reaches the active stages. Strong out-of-band signals, such as nearby broadcast or paging transmitters, are the usual cause of front-end overload, desensitization, intermodulation products and image responses, and keeping them out of the mixer prevents those problems. The preselector does not improve the receiver's ability to separate signals within the band; that job belongs to the IF and audio filtering.
Watch out The choice about broadband attenuation before the first RF stage describes a plain input attenuator or pad, which cuts everything equally; a preselector is frequency selective, attenuating out-of-band energy while passing the wanted band with little loss.
Preselector = pre-select the band: a front-end bandpass filter that keeps out-of-band giants out of the mixer.