In the sense reseller itself means to sell again. This means that if we managed to sell an item we will benefit. In principle, we might as well sell directly to consumers. If we can sell an item of course we can profit from the difference between buying and selling price.
So, Affiliate (affiliate marketing), is a program that offers commissions to us if we can sell a some product. Affiliate give the amount paid could vary, some offer 60%, 50%, 40% and so forth, of the total product we are selling. For example we sell product A for $ 100 and the affiliate program offers 60%, then the benefits that we can is $ 60