Patsplaining…

CLI=commandline interface (the command prompt)

The cat command is the UNIX command that concatenates files. You type “cat <filename> <filename> etc...” and it puts them together one after the other into one, which is sent out to the standard output. The standard output is usually the terminal screen. It’s probably most often used by people to quickly dump the contents of a single file to the screen, “cat <filename>”.

If no file name is given, the default is to take input from standard input, which is usually the keyboard. So if you just type “cat”, it will take whatever you type on the keyboard and send it to the screen without processing it, which is kind of useless, except in this weird case. 😄