The 50 ohms is the "characteristic impedance" of a transmission line. For digital computers, a square wave will travel on the PCB copper trace with a speed that is similar to a light beam in glass. It is slower than light in a vacuum
The impedance is determiined by the inductance per unit length and capacitance per unit length
Z = sqrt(L/C)
The speed of signals is
v = 1/sqrt(LC)
similar to speed of light
c = 1/sqrt(eu)
where e is permittivity and u is permeability