The items on display here could be posters, tools, car accessories or practically any other commodity. The concept of a 'shopping cart' is that you, as the consumer, add a number of goods to your 'cart' and then pay for them on the way out.
As long as you press the 'Add to Cart' button next to your selection, the values will be stored in your browser as cookies. This means that you can leave the site, even restart your computer, and when you come back (before the 28 day expiry date) the values will still be accessible.