Anything put in the way of flowing water causes a resistance to the flow (by definition), slowing it down and creating a back-pressure known as the differential pressure, delta-P or ∆P, or pressure drop. It is simply the dynamic (flowing) line pressure provided by the municipal waterworks or other pressure source, minus the gauge pressure at the outlet of the filter (or whatever). It is equal to the sum of all the many little resistances along the way from point A to point B: every centimeter of pipe or tubing, every fitting and valve with a reduced orifice, every particle or fiber of filter medium, even every change of direction with an ell or tee adds to the overall pressure drop of a system.