Headless Commerce is a way of designing the architecture of an ecommerce platform which separates the front-end (also called the head) from the back end ( also called the body).
The decoupled headless architecture gives greater flexibility and enables enterprise brands to make quick changes to the front-end customer touchpoint(s) without the need to disturb the backend logic. Similarly, thanks to the decoupled architecture any changes done to the backend do not disturb/hinder the user-experiences on the frontend.
For example, you can use the same backend logic and APIs and create a new customer touchpoint rapidly by adding a new frontend.