Not exactly a difficult concept. Actually, and this is the whole truth, it has to do with showing a particular section of the frequency range an increased or decreased impedance so as to filter it out and separate it from the rest of the range. To do this, you either create a short path out of the intended signal path for this signal so as to dump it somewhere else, or bypass an in line resistor with a capacitor so as to make that range of frequencies not affected by the resistor while all others are. You will achieve different results depending on how and where it is done, for instance impedance filters as part of an active gain stage have a rolloff of 12dB/oct as opposed to a simple passive filter which has an aspect of 6dB/oct rolloff. Digital in the box EQ's can approximate this, but only a tuned analog circuit can really pull it off in a pleasing way... but yes, you cannot simply "remove" a frequency, only "attenuate" them, and typically the attenuation is sloped or humped and not a single pure frequency, other frequencies in the near vicinity are affected as well. Don't forget that when you attenuate a frequency by way of hump/valley method as opposed to slopes (HP/LP) that you are altering the harmonic structure of the source material. This can be a potentially dangerous thing.
This is all well understood common knowledge.
