Soil resistivity indicates how strongly the ground opposes current spreading through it, expressed in ohm-metres. It varies enormously: from under 10 Ω·m in damp clay to over 10,000 Ω·m in rock or dry sand, and at the same site it changes with the season and with moisture.
It is the starting point for any earthing design, because it determines how many rods, of what length and at what spacing are needed to reach the target resistance. Measuring it before construction saves rebuilding the installation afterwards.
The standard method is Wenner's, with four equally spaced electrodes in a line: current is injected through the two outer ones, voltage read between the two inner ones, and resistivity comes from multiplying the measured resistance by 2π and by the spacing. Varying that spacing probes progressively deeper layers.