I just read about it and there are different forms of PR.
You could end up with eg if party A got 30% of the vote, Party B, 31%, Party C, 20% and Party D, 19%, then,
in the House of Commons you would have 30% A MPs, 31% B MPs, 20% C MPs and 19% D MPs.
So that when they voted to pass new laws, they would be voted on by a more representative (of who the electorate supports) mixture of MPs.
Or you could vote by marking your order of preference for candidates in your constituency and if the one who wins doesn't get at least a 50% majority, they take into account 2nd choices etc until they get a majority winner.
And there is another version where you put down 2 choices.