Basically, you buy a bar, go to the website to enter the code on the inside of the wrapping, and if it's a winner you can get a 1-month NowTV pass. It's limited to a maximum of 4 passes per customer and only 1 of each type of pass, i.e. a maximum of 1 pass each for TV, Movies, Kids, and Sports.
I've entered twice and won twice. Great for free TV. Not great if you're (supposed to be) on a diet

The offer runs until 15th July and you have until April next year to redeem your vouchers.
On a related note, NowTV are also offering their TV and Movie passes at a 66% discount, down from €15 to €5 per month, for two months. That offers runs until 2nd July 2018. New customers only, but in my experience a new customer is anyone who doesn't have a current pass.
So if you win a pass in the Nestle promotion, your best bet is sign up to the €5 per month for 2 months offer first, and then add the code for the Nestle pass in the second month. That way, you're getting three months TV or Movies for a total of €10.