From the same two viscometer readings used for plastic viscosity, it is the 300 rpm reading minus the plastic viscosity, in pounds per 100 square feet. A higher yield point suspends and carries cuttings in the annulus. It also raises annular friction, so it is part of the equivalent circulating density.
It is a model number, not a direct measurement of the stress at which the mud first starts to move. That starting stress is closer to the gel strength. Raising the yield point to clean the hole has a limit: the extra circulating density can fracture a weak shoe.