Linux commands communicate via the CLI. It can print information out to the terminal, and accept input from the keyboard. It has 3 streams (connections) to the outside world viz. input, output & error.

Standard Input : Mainly Keyboard Input, but redirection of one command output to another command; reading from file can also be called Standard I/O

Standard Output : Mainly Terminal Window, but redirection to file can also be called Standard I/O

Standard Error : Initially attached to the command window, but it has separate channel intended for printing error messages.

You can attach one or more of these three streams to a file, a device, or even to another program.