I/O is short for Input/Output. This is what enters your machine, pending processing -- and what exits your machine, processed. If you are programming, and you instuct the machine to echo something you are generating OUTPUT. The stuff you are instructing the machine to say is considered INPUT.

                             (cpu sez: I wanna make this 4, yes.)
                          ___/        ___
input (2 x 2)--->--->--->|CPU|       |VID|--->MONITOR: 4
                         |___|       |___|
                           |           |

This example: you send 2*2 (input) to the CPU, beg and plea for it to calculate, it calculates, and goes through neseccary stuff, and the stuff you get (2*2=4) on the monitor is your output.