Headless describes the approach of separating the frontend presentation layer from the backend ecommerce solution that manages all commerce functionality.
A headless CMS (content management system) also separates the frontend and the backend, so it stores files but also allows for editing and equips developers with an API to add content into the frontend.
Moving from a commerce-led to a content-led approach, a headless CMS lets you build separate frontends (aka heads) for each channel that displays content, so you can use a single CMS to create channel- and user-specific content. However, you’ll have to build your own frontend layers, as the CMS only provides the backend (aka body) layer.