A normal value is about the weight of a column of formation brine from the surface, often near 0.45 psi per foot. Overpressure is higher, common where compaction or hydrocarbon generation has trapped fluid that could not escape. Underpressure shows up in depleted reservoirs that have been produced.
Mud weight has to stay above the pore pressure, with a trip margin, or the well kicks. It is estimated from offset wells, from logs, and from seismic velocities, and it is confirmed while drilling by the way the hole behaves. A pore-pressure plot is drawn against true vertical depth. The fracture gradient is the upper bound of the same plot.