Basically you log in to Quidco, search for the retailer you are thinking of buying from, then click the link on Quidco's page to visit the website. Going via Quidco, you make exactly the same purchase on the same website that you would have anyway, but you get some money back (paid to you by Quidco).
So for example, instead of booking your transfer at the A2B Transfers website, go to quidco, search A2B, click the link and you end up at the same A2B site, but you'll get some of your spent money back. It's good for many things and especially good if you are thinking of getting Sky/Virgin Media/mobile phone/insurance where the cash back can be anything up to £150 - just for visiting via Quidco rather than going directly to the same site.