• Short version: press it if you want the aircraft to slow down to approach speed.

    The approach phase reduces the automatically selected airspeed (magenta airspeed, officially called managed airspeed) to the approach speed. It also arms the go around mode for when you move the thrust levers to maximum takeoff goaround (TOGA), deletes the cruise altitude and a few other things.

    The approach phase activates automatically if you are flying along the route in NAV mode and the point at which that will happen is indicated by a magenta "D" with a circle around it at the so called deceleration point. When you are flying in heading mode though this doesn't happen automatically and you have to press that button to manually activate the approach phase. The deceleration point is then white in the navigation display and no longer magenta.

    And if you're flying around with no route or decided to land somewhere else this button helps you to reduce the airspeed without the need to manually select an airspeed.