Great question. High voltage power lines are used to move electrons from one place to another – this is called electrical current. The electrical current is then broken down to medium voltage and low voltage lines where it can connect to buildings and houses to supply electricity at safe levels.
However, for electrons to move from place to another, there needs to be something to make them want to do that. This is called “potential” – and it basically means anything that will create a path for the current to move. This is like a bird on a wire – there is no path for the current to travel through the bird, so it has low potential.
Usually if a human touches a voltage line, they are standing on the ground, so there is a path for the current to travel through – from the wire, through the person, to the ground. There is high potential and the human will be electrocuted!
Technically, if a person could just stand on top of wire without touching another wire/the pole/the ground they wouldn’t be electrocuted (they would be just like the bird). Although that would still be a very dangerous thing to do try! In the same way, if a bird is standing on wire and hits it wing accidentally off another wire, it will create a path for the current and it will be electrocuted.