Facebook retargeting (AKA Facebook remarketing) is a form of marketing that involves using Facebook ads to re-target people who are already somewhat engaged with your brand. You do this by placing cookies on their computers and that way tracking the things that they're interested in.

For example, if someone spends a little bit of time on the checkout page of your website, with a specific product (or category of product) in their shopping cart, you can then subsequently show them that same product or a similar one.