The app shows a compass at your position on the map. The map always points North, this way what you see in real life in front of you will be aligned with the map on your phone.

The compass shows the right way even if you hold the phone in a vertical position. Tilt your phone upwards and the map also tilts showing you the way in 3D.

Knowing the map stays in the corect position however you rotate helps you orientate much easier then a simple compass because it lets you match landmarks in your sight to their representation on the map.