tl;dr: it is an online games genre.
The .IO games are a real-time multiplayer games of which the name ends in "io" (not really always :). They typically involve battles between players in an arena.
Players can earn points by killing other players.
The top players are listed on the leaderboard in the game.
The most famous .IO games are Agar.io and Slither.io.
Check yourself at our iogames list.