Dynamic Search Ads (DSAs) let you target entire websites, or portions of them, without having to create keywords and ads for all the possible matches. Instead, whenever a relevant user search occurs, the system dynamically generates an ad with a headline based on the query, and with text based on your most relevant landing page. The ad enters the auction and competes normally. So you get better results from broader exposure for more of your in-stock inventory, without making any changes to your existing keyword campaigns.
In addition to enabling you to broadly target an entire website or domain, DSAs can also serve as a "catch-all" or backup option for your existing campaigns and keywords. You can set up keywords, ad groups, and ads as you do today, and then add a DSA to catch other queries that don't match your existing campaign setup.