Thanks anon,
That is an approximation of pi.
If you roll a wheel one complete rotation then it will travel exactly $${\mathtt{\pi}}$$ times the diameter of the circle in distance.
pi is an irrational number which means that it cannot be expessed exactly in digits.
It is approximately 3.14158

[I give credit to flflvm97 for first using this Gif on the forum. Thanks fl :) ]