What is this port labeled "ONT" and what does it do?

This image is from the back of an AT&T uverse 3801hgv modem. What the purpose is of this port.

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Looks like it is for connecting to a Optical Network Terminal (ONT). What is a ONT then?

From dslreports:

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
• Video

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 telephones