If a live power line touches the ground it will form a pool of electricity in the local vicinity (on the ground). Depending on how much power was running through the wire in the first place, this pool of electricity may travel as far as 20+ meters. The further out of the pool of electricity the less the voltage. It is this voltage gradient that becomes a concern for attending emergency service personnel, because of the risk of electrocusion (as a result of the voltage gradient).
The touch potential refers to the voltage gradient between where you touch and where you stand – if there is a gradient difference between where you touch and where you stand, you will get electrocuted.