ASCII art is an art of combining characters laid out to form a pattern on screen. Its nice to be used in a text mode environment, such as ssh, irc, and other similar environment, just to get yourself not too boring 🙂
You can go to this site, http://www.network-science.de/ascii/, where you can generate different ASCII art to form a text in different size and font.
This is an example
You can change welcoming message of ssh in the file /etc/motd/. Try relogin and you will see the welcoming message