A JAMstack site is characterized by content living in a git repo alongside code and templates. This allows them to follow a generate-deploy cycle where a site is rebuilt each time a content or code change is made. Because of the git based workflow, every change is a valid commit to git and by extension deploys are immutable. The ability to revert is a key distinguishing factor of the JAMstack. In the past, it was commonplace to deploy sites via FTP, which meant your server and files within it were mutated with each new deploy. Mutable deploys present a host of challenges. These include ever changing and unpredictable server environments and issues with invalid caches. JAMstack sites do away with this by taking advantage the ability for modern CDNs to invalidate your cache completely.
For more on what makes a site "JAMstack", check out the comprehensive guide to the JAMstack published by O'Reilly Media