This term is shorthand for "input/output". It's a common IT phrase, used to describe any program or device which transfers data between a computer and a peripheral device. It is stated by computers experts that every data transfer is an output from one device and an input into another. Therefore all the common peripheral devices connnected to home computers can be categorised as either,

Input device - gamepad, joystick, keyboard and mouse.

Output device - printer and monitor.

Input / Output device - modems, network cards, floppy disk, writable CD-ROM and zip drive.