When you're using a GUI (in our case, a desktop environment), you can access the CLI through a terminal window, or terminal.
The procedure for bringing up a terminal varies depending on which desktop environment you are using.[9]
If you're using GNOME, select the Applications menu from the top of the screen, then the Accessories submenu, then the Terminal application.
You can also press Alt-F2 and type gnome-terminal at the prompt.
If you're using KDE, select the K menu from the lower-left corner of the screen, then the System submenu, then the Konsole application.
You can also press Alt-F2 and type konsole at the prompt.
If you're using a desktop environment other than GNOME or KDE, you'll need to consult the documentation for that environment.
[9] Desktop environments, including GNOME and KDE, are discussed in Section 1.2, “Graphical user interfaces (GUIs) for Linux”.