When a fault current disperses into the ground, the potential of the soil is not uniform: it falls rapidly with distance from the injection point. That gradient is what creates the hazard.
Step voltage is the potential difference between two points of the ground one metre apart, roughly the distance between the feet of a walking person. Touch voltage is the one appearing between an earthed metal structure and the ground one metre away from it, that is, between the hand and the feet of whoever touches it. Both can be dangerous even when the earthing resistance is low.
What is judged is not the voltage itself but the current that would pass through the body during the time the protection takes to clear the fault. IEEE 80 and IEC 61936 give the permissible limits as a function of that time and of the person's weight, and from these come the mesh design, the tighter meshing at corners, and the gravel layer, which raises the contact resistance of the foot and so raises the tolerable limit.