You don't need a voucher.
You get up to £10 as a 50% discount off food and soft drinks. So if you go to a restaurant and spend £18 on a pizza and a couple of cokes, you'll be billed £9. The restaurant claims the other £9 back from the government.
It's Monday to Wednesday only, but you can use the scheme as often as you like. Alcohol not included. If you have a slap up meal for two and the bill is £100, you still only get £10 per person off the meal.
But it does mean that something like a McDonald's meal typically costing £5 will be £2.50.