Clickbank is a marketplace where owners of (mostly) digital information products can list their products for sale. Anyone can sign up to sell any product offered on clickbank, so if you have a web site about raising chickens, you could find chicken coop plans, health guides, husbandry courses, etc to sell on your web site.
You might even find an appealing clickbank offer and build a web site devoted to selling it. Both of these methods are called "Affiliate Marketing" because you are affiliated with the owner of he product and have a legal right to sell the product. Click bank handles all the transactions and sends you your commissions.
If you decide to expand your footprint by building a few blogs, facebook pages, and the like about the product that point to your affiliate sales page you are building a "sales funnel" for that site.