Imagine for a moment an election with two candidates - for the sake of argument one labour one conservative.
The election gives a result of 60% to labour, 40% to conservative - so the voters get a labour MP.
Now imagine again, but this time at the last moment another candidate comes forwards - a LibDem. This (for the sake of argument) splits the previous labour vote, but doesn't change the conservative vote.
The result this time is 25% libdem, 35% labour, 40% conservative - so the voters get a conservative MP.
See what happened? An extra candidate ran, they didn't win, but they changed the result !
Some people claim FPTP is fair because the person with the most votes wins - but this example shows that just isn't so... AV demands the winning candidate gets more than 50% of the vote. A candidate who only has 40% support could never be elected, regardless of how much the voters are initially split between the other candidates.
So say #Yes2AV !
(ps. Feel free to swap the party names around - this isn't a party political point, it is a voter/representation/democracy point)