The command line can be used to pass parameters such as variables or commands to the Quake engine. For example the display resolution to use, the map or mod to load, making the engine act as dedicated server and so on.

There are multiple ways to use the command line. What is available for you depends on the operating system you use.

In most graphical window systems you can rightclick the executable and create a shortcut or launcher. You can then edit its properties to pass a custom command line.

Alternatively, especially if you change the command line a lot, you can use a terminal or command line prompt (some people on Windows like to call it a dos box or dos window) to run the game. There you can easily add command line parameters.

On Windows you can press Win+R and enter cmd to get a command line window.