It's latex notation and should render like http://arachnoid.com/latex/?equ=p%5E%5Cmu%20%3D%20m%20u%5E%5... . It's a bit obtuse to be using it on a non-physics/maths forum.

mu is being used as an index (mu=0,1,2,3) on the components of the vectors p and u, m is a scalar representing rest mass.