You can configure discounts in 'Settings/General settings/Discounts'.

Discount buttons enable to apply a discount with a single click:
The discount label is the name of the discount button on your sales interface.
Discounts can be in percentage or in value.
The value is either a rate (-10 for -10%) or an amount (-5 for -5 EUR).
The discount type (% or value) must be selected in the last drop-down menu.
You can hide or delete an existing discount.

You can apply line-item discounts (select the product in the sale and apply the discount) or sale wide discounts (click on the discount button below product keys in the sale). More information.

Line-item discounts are included in the final price of the product.

Sale wide discounts are linked by default to a single product (ID 0 'unidentified').
To better track your discounts, you can link each discount code (or all) to one or several products from your product database.
Ex:
If we create a 10% discount named 'Black Friday',
we can create in the Products section of your account a product named 'Black Friday Discount',
and enter this product ID in 'Settings/Discounts' for the associated discount.
You can then track your sale wide discount 'Black Friday' by consulting the sale data of the 'Black Friday Discount' product.