I/O is a general name for any operation in which your code interacts with the outside world. There are many things that count as I/O, and there can be plenty of I/O that is internal to your software, especially if your application has a distributed architecture.
The increasing use by tech companies of the .io Top Level Domain (TLD), for example http://github.io, can be partly attributed to it standing for I/O, but that is not its real meaning. As is the case for some other TLDs, it is actually a country code. Other examples include .ly for Libya and .tv for Tuvalu (which, like the neighboring Kiribati, may soon be submerged beneath the Pacific Ocean due to climate change). The TLD .io is intended for the British Indian Ocean ...
