You may remember from the installation chapter the feature known as virtual consoles, which allows you to have a number of simultaneous, command-line login sessions going at one time.
Originally, when you log in, you're on the first virtual console, and can switch to one of the others with the key sequence Alt+Fn with n being the number of the desired virtual console (yes, you can log in as the same user more than once). This can be useful since, if you have the root password, you can be logged in at one virtual console as root, and another as a regular user, depending on what you're trying to do.
Initially, you most likely have six available virtual consoles, with the option to configure more if you really need them. However, if you're actually using all six virtual consoles on a regular basis, you might reconsider just what you're trying to do. Besides, once you start a graphical desktop session, there's little need for more than a couple virtual consoles anyway.
While we haven't discussed device files yet, your virtual consoles can be distinguished by the name of their corresponding special device file. Typically, these have the names /dev/ttyn, with n being one of 1 through 6. In case you need to know, you can see the device file name of your current virtual console with the simple command:
$ tty /dev/tty3