I/O is the Abbreviation for Input/Output. Strictly speaking any device that does anything will have input and output. A seesaw, for example, utilizes the energy from childrens legs (the input) to rock back and forth (the output) on a fulcrum. But the term is mostly used in electronics, especially as it pertains to computers or any kind of logic functions, but also with audio and video equipment. Computers have all sorts of I/O, from serial ports, to SCSI, to monitor and keyboard ports. Audio and video equipment is obviously all designed with the ability to get signals in and out as well. These inputs and outputs, when spoken about collectively, are sometimes called I/O for short.
I/O input output signal processor mixer ports keyboards audio equipment