A full stack developer is someone who can work on several layers of a Web or Software Application. They are responsible for creating user interfaces, server-side logic, interacting with databases, designing APIs, and integrating various parts of the application. You don't have to learn all the programming languages to be a full stack developer. Rather than that, developers usually gain proficiency in a specific technology stack or understand the functionalities of the various parts used in it. For instance, HTML/CSS/JS can be used for the front-end, and a backend language and database technology can be used. They have a wide range of skills that can be used to develop applications end-to-end.