A voucher code is a code for an immediate discount from the overall purchase price, such as free delivery. The code must be entered during the check out process on the retailer's website and should show a discount before you commit to buying the product.
Retailer offers are generally not exclusive to Quidco and are there to draw your attention to an offer or reduction that already exists on the retailer’s site e.g. a Blue Cross sale, to show you that you can take advantage of these offers and reductions and still earn cashback, where applicable. These don't necessarily offer a further discount than you will see when you visit the retailer's site, but the products in the offer will be eligible for cashback.