Dynamic content is a different way of delivering content – instead of having static calls-to-action, dynamic content would personalize the offer to the visitor using whatever metrics they can gather from them just visiting the site. If a user has already visited your site, or maybe even registered in some way with the site, you could customize their user experience even further to keep them engaged with your site and content!
It's the process of displaying different messaging on your website or in your online advertising (e.g. Facebook’s Dynamic Ads) based on information you have or your enriching about your target audience.