Ultimately, you have an online store that is run by Amazon. You find products at a discounted rate (clearance, stacking sales, thrifting, ect.), ship them to Amazon and mark them up to the prices they are currently selling for. When a customer purchases one of your items Amazon ships your item from their warehouse. Amazon then takes a cut for storing, shipping and marketing your product. You receive the remaining profit.
This really is a super easy business model, and can be really rewarding to run! It is much easier to start than an independent e-commerce site because you have the traffic and credibility of Amazon. You can easily make $500 or much more with an Amazon FBA business.