Very simple, TTY is an abbreviation for teletypewriter. Do not understand? You for sure have used .. If you use linux .. if you do not close that uses your internet explorer .. and look for a window to play. If you use MAC makeup and go for this PC formats. Jokes aside, TTY is when you use the shortcut key

 CTRL + ALT + F # 

, or even when you open a terminal on your OS.