Tau represents a circle constant of 6.283185… it is used to relate the circumference of the circle to its linear dimension. In recent years there has been a growing movement that Tau is superior to Pi for showing this relationship. The TAU series is my classic reproductions of vintage Big Muff Pi circuits with switchable mids profiles.
If you want more information on how Tau is superior to Pi in math take a look at http://tauday.com/. If you want more information on Vick Audio's Tau Series pedals look below.