Sorry about my initial reply, I got confused. Let's start over:
The tab "discount" is money taken off of the price of your phone and put onto the tab. It's not really a discount, it's more of a 24 month interest free loan. What happens is that the amount you decide to put onto the tab, will be divided into 24 monthly payments and added on top of your monthly plan.
Take a look at a short video explaining the tab here.