The JAMStack is an architectural pattern (sort of) for building apps that moves away from the traditional web server model and focuses on serving static files from “the edge” (data centers physically close to your computer).
JAM stands for Javascript, API, and Markup. Javascript: all code is in JS, for both frontend interactivity and API requests and handling. API: instead of a monolithic web server, all backend functionality (authentication, users, etc.) is served via APIs. Markup: your whole site is just a bunch of HTML files served statically.