Looks like it is for connecting to a Optical Network Terminal (ONT). What is a ONT then?
An ONT is a media converter that is installed by Verizon either
outside or inside your premises, during FiOS installation. The ONT
converts fiber-optic light signals to copper/electric signals.
Each ONT is capable of delivering:
- Multiple POTS (plain old telephone service) lines
- Internet data
The wikipedia article has some more information here
An ONT (optical network terminal) is used to terminate the fiber optic
line, demultiplex the signal into its component parts (voice
telephone, television, and Internet), and provide power to customer