Retargeting, or remarketing, is a way to follow your audience across the web and to target them with appropriate ads and messaging. By embedding a pixel (a small piece of code) on your website, or link, you are able to track visitors and continue your marketing with them in other places.
Imagine you run a Facebook campaign; now, you can 'pixel' all the traffic you previously sent to Amazon (or elsewhere) and retarget it with another ad or another offer. Also, you can collect the data behind the traffic, e.g. demographics, locations and operating systems, etc.