With the introduction of e-commerce, there was also the emergence of coupon codes (which corresponds to coupon that you can always find in physical stores).
On the Internet, it's the same : the site offers a coupon code, valid for a specified period (long or short) and valid on the entire site or only a part (a type of product, a specific brand, ...).
This allows you to save money easily.
Generally, promo codes are sent by mail and/or are posted on social networks and should be added just to the cart.
Promo codes have a limited validity period and allow you to save money depending on the time, brands, and sites.