An Affiliate Program is one that pays you money for selling other people's stuff. Say for example Amazon, which is one of the biggest affiliate programs out there now. They sell books and other stuff. If you sign up with them as an affiliate, i.e. someone who is interested in selling their stuff for a commission or percentage of the sale price, then each time your affiliate link (which they provide for you to use) ends up resulting in a sale, you make money. People who are interested in buying the advertised products click on your link which carries them to the page that sells the item they want. Once they follow through and make a purchase, you get paid.
there is free affiliate programs out there you can join and start earning money with. Such as... moreinfo247.com clickbank.com cj.com