io games are online multiplayer computer games that take place in real-time. The games in this genre are easy to play for both regular players and people who don't play games very often. The fact that most io games are free to play also makes them very easy to get to. But the fact that they are easy to find and play doesn't mean they are easy to learn.
The domain io is the internet code for the British Indian Ocean region, which makes the type of game stand out right away. But, interestingly, io games have nothing to do with this area.
The answer isn't very exciting. This name was used by the first io game, Agar.io, and is still used by it. Its popularity led to a lot of simple graphics and rules multiplayer games that used the same name to show how similar they were to Agar.io. As time went on and more and more games of this type were made, the io code became associated with them.