What is the requirement of input output interface?

For it to be an input device, the requirements are for the user to be able to input information into a device or computer system. (keyboard, mouse)

Output devices need to show and display information aswell as produce information. (monitor, printer)

A touch-screen mobile phone would be a good example for an input/output device, you touch the screen to get it to do something, at the same time you gather the information from the screen.