Working across the product
Frontend developers build what people see and use; backend developers build the server code that handles data and rules. A full-stack developer works across both, perhaps building a checkout screen and the server code that records an order.
The term describes breadth, not expertise in every tool in the tech stack. Full-stack developers often have deeper expertise in some areas, and larger teams may also need specialists in accessibility, security, data, and operations.
One feature spans both sides
Even a small change may affect a screen, a server, and a database. A full-stack developer can work across these pieces while specialists review changes in their areas.