The word affiliate can serve as either a verb or a noun. As a verb it means to associate, probably at least somewhat formally or legally, with another person or an organization.
This is done with the intent of achieving a mutual benefit of one sort or another. Whether or not the people or organizations actually achieve that benefit is another question and beyond the scope of this project. As the expression goes, affiliate beware. Alright, the expression is buyer beware, but the same principle holds.
As a noun, an affiliate is the person or organization that has affiliated with an organization.
For example, I have signed an affiliate agreement with Amazon. As a result, if you buy a book through one of the Amazon links on this site, including the bookstore, I get a referral fee from Amazon. Consequently, it would make me very happy if you bought stuff from Amazon through the links on this site. This concludes the shameless plug for Amazon and my referral fees.