Ok, so, an example. There are two parties, lets call them A and B, and they both put forward 5 members, and they are elected through our current system, where every voter has 3 selections to make. Lets say A has around 60% of the support and B has 40%. The voters for A, like they do in our system, vote pretty randomly, then they will all get about 3/5 of their party's supporters as that is the maximum people can select, so all their candidates will get about 36% of the vote. The voters for B, will naturally do the same, and so their candidates will only get around 24% of the vote. Since every A candidate got more than that, A will elect 5 MPs. But if it were a single transferable vote system, then A would end up with 3/5 of the vote and get 3/5 of the seats, because their votes exhaust as they elect members, making it a much fairer Parliament.