5.1.1. What are standard input and standard output?
Most Linux commands read input, such as a file or another
attribute for the command, and write output. By default, input is
being given with the keyboard, and output is displayed on your
screen. Your keyboard is your standard input (stdin)
device, and the screen or a particular terminal window is the
standard output (stdout) device.
However, since Linux is a flexible system, these default
settings don't necessarily have to be applied. The standard output,
for example, on a heavily monitored server in a large environment
may be a printer.