This is a slot for an SFP or SFP+ module. The actual fibre optic cable is then plugged into this module.
https://de.wikipedia.org/wiki...
https://www.digitec.ch/de...

I assume that you have had Swisscom with the Internetbox 2 up to now, for example, or? This router is an XGS-PON router and is not compatible with the old cable and the configuration of your fibre optic socket. Ask your ISP if your connection can be switched to the new technology.