When it comes to what is full stack developer, it's essential to understand what they do throughout an application. This type of developer can develop the front end interface that users interact with and the back end server components running without the user's knowledge. This involves making interfaces, writing application logic, handling databases, connecting APIs, testing features, and bug fixes. They could also work with designers, product managers, and other developers. The job roles vary from company to company, but the concept is to be able to contribute to multiple layers of a web app.