The CLI is the black window with white text you see in every hacker movie: the terminal. Instead of menus and buttons, you type an instruction, press enter, and the computer does it.
It looks intimidating, but there is no magic in it. npm run dev is not code, it is a typed instruction, the same way "search flights to Milan" is an instruction. Developers prefer the terminal because typing is faster than clicking through menus, and because typed commands can be saved, repeated and automated.
Why you keep hearing it
Because several of the most capable AI tools ship as CLIs first, before they get a pretty interface. Claude Code, for example, started as a CLI. When a tutorial says "install the CLI and run this command", it means the tool lives in the terminal rather than in a browser window.
What it means for you
Two things have changed for non-technical people. First, you can now copy and paste terminal commands from a guide without understanding every character, and mostly things just work. Second, and better, AI assistants can run the terminal for you. A growing amount of "technical" work is now you describing what you want in plain language while the AI types the commands. The black window is much less of a wall than it used to be.