This wiki is incomplete.
Input-output is the concert of what is taken in (input) and what is produced (output). This can be used in many contexts, from functions to algorithms to computers.
Basically, before knowing the terms input and output, we have to know what is computer hardware and software.
Computer hardware: Whatever physical computer device you may touch is computer hardware. Computer hardware is also divided into two different parts that are called input devices and output devices. Whatever device is used to feed command to the computer is called input device. Whatever device provides the results of commands and processes is called output device. For example, a computer keyboard which inputs commands is an input device, whereas a computer monitor that displays what the keyboard typed in or the result of the input command is output device.
Computer software: Whatever you see on monitor screen as a result or command or process is computer software. Similar to computer hardware, input devices give commands called software input command, and the output device provides the results on monitor screen or non touchable (virtual) form that are the outputs of the software.