A desktop environment. It includes most of the software you directly interact with once you boot up your computer (session manager, window manager, task bar, etc…) Some of the user friendly DEs used in Linux include GNOME (default for Ubuntu), Cinnamon (default for Mint), KDE, Xfce…

If you have no DE at all, you just have a shell and you can interact with your system only through command lines.

But in Linux, there’s a middle ground where it’s also possible to have only some of the software that make up a DE while keeping your system somewhat minimal. For example, you can login through the shell (and not use a session manager) but then run “startx” if you use X11 or a window compositor like “sway” if you use Wayland and still have a graphic session.