So you're in the middle of working on something and you decide you need to do
something else. You could just drop what you're doing and switch tasks, but
this is a multi-user system, right? And you can log in as many times
simultaneously as you want, right? So why should you have to do one thing at a
time?
You don't. We can't all have multiple keyboards, mice, and monitors for one
machine; chances are most of us don't want them. Clearly, hardware isn't the
solution. That leaves software, and Linux steps up on this one, providing
“virtual terminals”, or “VTs”.
By pressing Alt and a function key, you can switch between virtual terminals;
each function key corresponds to one. Slackware has logins on 6 VTs by default.
Alt+F2 will take you to the second one, Alt+F3 to the third, etc.
The rest of the function keys are reserved for X sessions. Each X session uses
its own VT, beginning with the seventh (Alt+F7) and going up. When in X,
the Alt+Function key combination is replaced with Ctrl+Alt+Function; so if you
are in X and want to get back to a text login (without exiting your X session),
Ctrl+Alt+F3 will take you to the third. (Alt+F7 will take you back, assuming
you're using the first X session.)