Resistance from the live to earth, via the supply cables and transformer.
Using ohms law, you can determine the fault current in amps, that would flow if a dead short was introduced. This is the current that would flow down the earth to the substation, and back up the live to your fuse, as the fuse blows.
The lower the earth loop, the higher the PFC (prospective fault current).
The higher the PFC, the quicker the fuse will blow.
We need to therefor know the earth loop (Ze at origin, Zs at circuit end) to determine weather the fuse/mcb will operate quickly enough to give disconnection with in the time stated in regs (0.2, 0.4 or 5 seconds, depending on circuit type and location).
A TN-S or TN-C-S system will give nice low resistances, the REC state that for a TN-S system, the max Ze they will provide will be 0.8ohm. For TN-C-S it will be no more that 0.35ohm.
For a TT system, the earth rod will generally give a higher resistance, and therefore will not provide a high enough fault current to operate the fuse/mcb quickly enough to conply to the regs. This is why on TT systems, an RCD is essential.