At the top right of the screen tap on you name and tap Settings in the menu, then on the settings page tap Discount codes.
Discount code settings is where you create and mange discount codes. These codes can be used for your events and stores to offer discounts.
The discount can either be a fixed amount or a percentage off. If the apply to all tickets option is selected then no code needs to be entered to have the discount applied. If it's not enabled then at payment the person will need to enter the name of the discount code to apply the discount.
In the example shown an Early bird discount is being created, this discount will only work during September.
The Uses field can be used to limit how many times a discount code can be used before it runs out. For example when used with the apply to all option you could create a special where only the first 100 people to register get the discount.