A headless CMS manages content — articles, products, pages — and serves it through an API, without imposing how it is displayed. The "head" (the website, the app, the kiosk screen) is built separately and simply consumes the content. One source of truth can feed a site, a mobile app and a newsletter at once.
The trade-off against WordPress-style suites is flexibility versus out-of-the-box comfort — the decision grid is in choosing a headless CMS.