Well, this new feature is added to Android 7.0 Nougat. This new feature gives a user the ability to run apps side-by-side or one above the other. To set up the new feature, tap the Overview button (square icon below the screen) and then choose which of your recent apps you want to see.
You just need to hold and drag that app’s title bar to the top or left of the display. Once done, simply choose another running app to appear alongside or under it.