What does Fullstack mean?

In a Web Application some of the code to runs in your browser, also known as the "Front-end" and some of the code runs on a server, also known as the "Backend".

"Fullstack" is often used when talking about a "Fullstack Developer", which is a developer who knows how to code for both the Front-end and Backend.

Or it's used when talking about a "Fullstack Framework" which is a Web Development Framework like Rails which provides the functionality to create the Front-end and Backend in the same codebase.