It was a brick walk - up with three apartments stacked on top of each other in a block of similar buildings. (bombmagazine.org)
This feature was introduced in Android 6.0 and allows a notification to pop up on top of other apps so you can quickly check it without leaving the foreground applications. (howtogeek.com)
For example, some apps can actually run over top of other apps. (howtogeek.com)