What is this round disk on my wall?
Most European homes use round junction boxes for electricity and other wiring, most likely similar to any of the round boxes found here.
Usually they're mounted in the ceiling (and used as a mounting point for a light fixture) but it's not unheard of to have them mounted in a wall.

The one in your picture has 2 screws with which the cover can be removed to be able to service what's inside.

It most likely contains electrical cabling but based on the height and it being located in a wall it could also be audio cabling as a preparation for wall-mounted speakers.