Remarketing allows you to show your ads to users who’ve previously visited your website — as they browse the Web and encounter Google’s ad network elsewhere.
From Google’s Forum:
Remarketing is a feature of interest-based advertising … Remarketing allows you to reach people who previously visited your website, and match the right people with the right message. You can show users these messages as they browse sites across the Google Display Network.
Here’s how it works: You add a piece of code (remarketing tag) to pages of your site that correspond to certain categories you want to promote. For example, adding a tag for “TV” on all of the pages where you sell TVs will let you later show relevant TV ads to everyone who visits those pages. Learn more about creating remarketing lists.
Search advertisers can use remarketing to create an integrated campaign strategy. After driving traffic to your site with search ads, you can then remarket to those users who reach your site by showing them tailored ads on sites throughout the Google Display Network.