This is exactly what you think it is
A Tone control
The Tone control of a Big Muff style pedal to be precise
A few other pedals use that too
In the build docs you will find quiet a handful of different versions you can build with this tiny PCB
Just note that this is a passive tone control wich will make your signal a lot quieter
So either add this to a circuit that is already loud as fuck or put a boost after this (this one or this for example)
Damn this was totally a proper info text
I am proud as fuck right now