Basically, it's THIS stupid:
Imagine that there are 4 parties standing for Manchester.
Party 1: Votes: 67424
Party 2: 67425
Party 3: 67426
Party 4: 67427
This is an extreme example, but it's just to illustrate what goes on in the wider scheme of things. If the above happened, Party 4 would gain a seat. The MP would be elected to the House. ALL OTHER VOTES WOULD BE DISCARDED. They would be meaningless. All that would count in the above scenario is that Party 4 = +1
The Scottish National party now have UNPRECEDENTED POWER with around 4% of the vote, compared to UKIP who have no power at all with 12%.