“Filters” are one of the audio modifier tools in a modular synthesizer that helps shape the tone of a sound. There are several basic types: low-pass, high-pass, band-pass, and notch. In this video, Chris explores each, allowing you to both hear and see the effect they have on the sounds patched through them.
The basic filter modes are illustrated on page 18 of Patch & Tweak; the discussion of filter modules starts on page 162. Get the book here.