Affiliates work to introduce their visitors to the merchant’s brand. They might write a post about a new product or promotion on the merchant’s site, feature banner ads on their site that drive people to the merchant’s site, or offer visitors a special coupon code. If people come from that affiliate’s site and make a purchase, that affiliate gets paid.
Clickbank, huh... now that's a name I haven't heard in a while! I remember messing around with them back in 2011 when I was a newbie. Never did make any money with them, haha, but I've seen plenty of people who have! I've just never been any good at pushing the weight loss stuff, which I'm pretty sure is where most people make their money with Clickbank.
Now as to your comment on traffic it is true, traffic is important. But the truth is, acquiring traffic is easier than converting it. Plus we have many posts dedicated to traffic generation on this blog. Simply check this post, this post or this post. But must importantly, the reason why people struggle with traffic is that they try to promote a sales page or a heavily biased piece of content. Nobody wants to share that, nobody wants to link to that, and as a result you don't get traffic and blame your lack of success on traffic (like you did in your comment above).
When promoting affiliate offers, just make sure you are fully aware of all the terms and conditions attached to your affiliate program. Some programs can be strict about how they allow you to promote their products. For example, some may limit you to banner ads and links only, while others will allow you to use paid advertising, but won't allow email marketing.