On a rotational viscometer it is the 600 rpm dial reading minus the 300 rpm reading, in centipoise. It goes up when the base liquid is more viscous and when the mud carries more solids. A mud weighted with barite, full of fine drilled solids, runs a high plastic viscosity.
A high value spends pump pressure inside the drill string, where it does little to carry cuttings, and it raises the standpipe pressure. It is a Bingham-plastic model number from two viscometer readings, not a complete description of how the mud flows at every rate.