Filter is a post-effect. It shapes the audio of any connected Source blocks by boosting or reducing the level of frequencies around an adjustable “center-frequency”, acting like a frequency-specific volume knob. It’s a “peaking” type filter.

adjusting frequencies like this alters the tone or character of the sound, allowing more low frequencies pass creates more bass, and higher frequencies more treble. Boosting frequencies of audible pitches creates resonance. A filter can only boost or reduce frequencies already present in the original audio.

In options, ‘Frequency’ is the “center-frequency” around which the frequencies will be affected. ‘Gain’ is the amount those frequencies are boosted or reduced.