Discount codes can be created in the Admin. Go to Settings → Discount and choose a discount code group, click on 'Discount codes'. Then click 'Add new codes' to open the discount code generator. Fill in the required information.
The prefix is a fixed combination of letters and/or numbers that precede the code. For example, if you enter 'DISCOUNT' here, all discount codes will start with the word discount. A code could, for example, be 'DISCOUNTKH5K6Z5WM73B'.