Front-end technologies are the software technologies, used to build the front-end (the user interface) of the apps.
Back-end is the part of the software systems, which users don’t see on their screens.
The back-end provides concepts, frameworks, libraries, and tools to build business logic, implement data processing and data storage, and exposе programming interfaces (APIs) for the front-end.
The back-end is the server-side part of the applications, where users and their data are stored and processed.
Full-stack development combines back-end and front-end development.
This is what most software companies do when they build software products.
Full-stack development requires end-to-end architecture, design, and implementation of both server-side and client-side components and integrating them into a single software system.