What is this button (pressed during approach) on the Airbus A320?
This is the TERR ON ND button, or Terrain on Navigation Display. An EGPWS feature that paints the ND screen with the terrain. The color coding indicates if the surrounding terrain would pose a hazard if flown close to. The push button operation is:

At 46:10 in the video the TERR mode was off, the captain turned it on, pointed something out to the first officer, and turned it off. Zurich (the destination) has areas of high terrain around the airport.
(Source) Click to view.
The color coding and example:
Click to view.