A bandpass filter is a circuit that allows signals within a chosen frequency range to pass while attenuating signals above and below that range. In RF systems, it is often placed near the receiver input so unwanted energy is reduced before it reaches sensitive stages such as the low-noise amplifier or mixer. This matters because receivers can be overloaded by very strong out-of-band signals even when those signals are not on the channel being monitored.
In cyber security and radio security work, bandpass filters help defend against jamming, accidental interference, and front-end desensitization. They are also used to improve selectivity in scanners, radios, and monitoring equipment, making it easier to detect the intended signal in crowded spectrum. A filter does not stop all interference, but it lowers the power of unwanted signals enough that the receiver can operate within its dynamic range. That makes it a simple but important physical-layer defense.