Hi MariaE.
You need to have a Tesco club/reward card which gives you points every time you use it when shopping at Tesco in store and on line in their web store.
These points then get transfered into vouchers which are sent out to you, every couple of months or so, to your home address. The value of these vouchers depends on the amount you spend using the reward card.
It soon adds up if you are like me. My weekly shop is easily over Ł100 and I get things from their on line stores as well and often they have double or triple points offers on many of their items