This option is useful if you have multiple monitors and you're playing a game in fullscreen. Often, you'll want to keep your mouse in the game window so you don't accidentally click away from the game.

Some games already confine your mouse. Other games, like Ori, don't, so it is helpful to have a tool like Reemap do it.

It's easy to get stuck with this setting. So as a reminder, you can enable Scroll Lock to disable Reemap.