The socket is a spot (hole) for a network cable in the junction box meant for the RJ-45 interface. In the junction boxes of the apartments, there are normally two sockets side by side, but only one of them is connected while the other is a back-up. Therefore, if you have only just moved in and are not getting your internet connection to work, try the other socket as well.

If the apartment has several junction boxes, it is possible that the connection is only online/in gear in one socket of one junction box.