Different types of the stuff we stand on have different resistances or conductivities. You can imagine solid granite is not as good a conductor as a farmers fie

Different types of the stuff we stand on have different resistances or conductivities. You can imagine solid granite is not as good a conductor as a farmers field of moist rich soil. If a particular section of earth has poor conductivity it and has a charge you can imagine the charge can't escape. Maybe it is charged from lightning, a downed power line, friction in a pipe, etc. this is called ground potential rise. If an area has low resistivity it can't create a steep potential gradient and the charge is spread out over the entire area. a ground grid of copper conductor is buried under substations and power stations so that the ground on the entire premise is at the same potential. When a power line touches the ground the current flows back to the source through the ground to complete the circuit. If the ground has high resistivity and current flows through it there is a voltage difference from one point to the next. If those two points happen to be your feet you are a better conductor than ground. By laying the copper ground grid no ground potential difference can exist as the copper connects the whole premise with low resistance conductor.