A command line is a user interface for interacting with software via text commands. They are most often seen as an alternative to the more popular graphical user interfaces (GUI). Because of this, some people use the acronym CLI — command line interface — particularly when drawing parallels and distinctions with GUIs.
Although command line interfaces are found in all kinds of software, the term when used without explicit context usually refers to an operating system's command shell. (e.g. Bourne shell on Unix, cmd.exe on Windows, etc.) This is the sense meant when this tag is used on this website.