You go onto the Tesco clubcard web site and you exchange £2.50 of vouchers into 600 airmiles (now called AVIOS).

You need to go to the AVIOS website to check out deals on flights, hotels, Eurostar, Disney tickets etc

Basically they are a form of currency, often you need to pay a bit of cash as well as your airmiles. Particularly if its flights as they have tax on them and the Gov doesn't want airmiles it wants cash lol.