In Plain Terms
Headless commerce separates the storefront (what customers see and interact with) from the commerce backend (catalog, cart, checkout, order management), connecting the two through APIs instead of the platform's built-in templating. That means the frontend can be built in React, Next.js or anything else, independent of the backend's native theme system.
What It Actually Buys You
Freedom to build a highly custom frontend experience (or multiple frontends — web, mobile app, kiosk — off one backend), and frontend performance that isn't constrained by the commerce platform's own rendering. It's real, but it's not free.
The Cost Nobody Mentions Upfront
Going headless means owning two systems' worth of engineering instead of one, plus the API layer connecting them — search, cart state, checkout, and preview all need to be rebuilt or carefully re-integrated rather than inherited from the platform's theme. It's worth it for a business that genuinely needs frontend flexibility a standard theme can't deliver; it's overkill for a storefront that would be fine with good theme customization.