It means that some people are excluded from this offer.
When a company makes a special offer, such as "Enter a code on our website to get a discount", they might want to exclude some people. For example, they might exclude people who work at the company, or they might exclude business users of the service.
The exclusions might be geographical, the discount might only be available to customers in the USA. People from other countries could be excluded.
Or they might want to exclude certain types of order. Perhaps they exclude orders over $1000 or under $10. They might exclude repeat orders (so you can only get the discount once). A company might provide a platform for other companies to provide a service, as well as providing a similar service, for example, "Zazzle" provides cardmaking services, but also provides a platform for other stationary companies. The offers apply only to Zazzle products, not to the products of other companies that use the Zazzle platform.
There are lots of possible exclusions. Dor details you need to follow the link.