From The RadioReference Wiki
Nearby pager towers or other strong transmitters can sometimes transmit signals so strong that they overpower a scanner, causing it to increase the AGC in the front end to the point where desired signals cannot be received. Interference of this type is especially common in large urban areas.
To compensate for this interference, many scanners provide method(s) to attenuate (reduce the strength of) incoming signals. Attenuation may be either global (affecting all channels, usually enabled with a switch near the scanner's antenna jack) or selective (affecting only specified channels, uusally enabled by an option specific to a particular channel). Details of any supported attenuation option(s) are typically described in a scanner's manual.
In normal RF environments, a scanner will perform best with the attenuation switch or option set to off, disabled, or 0 dB. Other settings should only be tried if you suspect that your scanner is being overloaded by exceptionally strong signals. Counterintuitively, sometimes reducing the overall RF input can help overall reception (by knocking the incoming signals down to a point where AGC no longer blocks everything).