Dynamic remarketing lets an advertiser:
engage visitors to the website by opening a chat session
show an ad to someone on his mobile device 24 hours after visiting the website on a desktop PC
show prior visitors to his site ads that are based on products or services they saw on the site
re-run an ad to increase the volume of people who will use it
Remarketing allows you to show ads to people who have previously visited your website or used your mobile app.
Dynamic remarketing takes this a step further, letting you show previous visitors ads based on products or services they viewed on your website. With messages tailored to your audience, dynamic remarketing helps you build leads and sales by bringing previous visitors back to your website to complete what they started.
Read more here: https://support.google.com/adwords/answer/6077124?hl=en