IO is an adult visual novel that also allows some freedom of maneuver around the world. As you progress through the story you'll have the opportunity to build a relationship with several women, and if you make the right choices, bring that relationship to fruition (sex..).
In order for you to be able to install this game, you need to enable (allow) installation of applications from unknown sources (Unknown sources) in the phone settings. Usually this item is in the Security section