Users interact with ecommerce websites through an user interface that consists of item lists, search, cart etc. This is the presentation layer also known as the frontend. Conventional ecommerce stores work on monolith architecture meaning the frontend is closely tied to the backend which coordinates tasks like maintaining the catalog, inventory, order processing etc.
This would seem to be simpler with all necessary components in a single system but then customization leads to complications, response to the market trends is slow and needs lots of testing etc
In the case of Headless ecommerce the front end presentation layer and the backend ecommerce / business layer are entirely independent of each other. This allows for incredible speed and performance, flexibility to design the architecture and focus more on delivering excellent user experience. In this case the requests are sent between the business and presentation layer through API calls.