Soil can be regarded as a set of interconnected capillary tubes which are able to hold water at a given suction (negative pressure; matric potential) depending upon the radius of the tube. The pressure potential due to a capillary of radius r is equivalent to the weight of (i.e. "force of gravity" on) the water column of length h.
When expressed in mm, h = 15/r. It is convenient to express soil water potentials in terms of the height of rise "h" which would be observed in a capillary tube having a radius resulting in the potential expressed ("equivalent radius").