Affiliate Marketing means that you sell a product, and you get a percentage of the total price of the product. Even big companies like Amazon have affiliate programs.
To make this easier, imagine this: You have a blog where you write dogs related stuff. If you have some traffic, you can put a banner on your website where you sell dog food from Amazon or other online shop. If your visitors click that banner, you will get from 5% to 20% in commissions of the total price.