The circular push button at the center of the FCU adds the Flight Path Vector symbol to the PFD (also called "the Bird"). It represents the airplane's actual lateral and vertical flight path, which are usually different from the direction the airplane is pointing. For lateral flight path, if wind is blowing from the side (crosswind), there will be a drift angle between the airplane's nose direction (heading) and actual flight path. The resultant lateral flight path is known as Track.
For vertical flight path, the pitch angle (angle between the nose of the airplane and the horizon) is not necessarily the same as its vertical flight path. The resultant angle between the vertical flight path and the horizon is known as FPA (Flight path angle).