The ONT — optical network terminal — is the small box where the fiber ends and your home network begins. It converts pulses of light in glass into the electrical Ethernet signal your router uses. If cable internet has a modem, fiber has an ONT: same seat at the table, different physics.
ONT vs modem vs router — three different jobs
The ONT terminates the fiber and speaks light. A modem does the equivalent job on cable's copper. The router is a separate function — sharing one connection among your devices and making Wi-Fi — that sometimes lives in the same plastic box as the ONT (an all-in-one gateway) and sometimes doesn't. When your provider says "plug your router into the ONT," that's the whole architecture in one sentence.
Where it lives and what it needs
ONTs mount where the drop enters: garages, utility closets, basements, or a weatherproof box on the outside wall. It needs two things — the fiber, and power. The glass carries no electricity, so during an outage the network may be fine while your ONT is dark; a small battery backup or UPS keeps internet (and fiber-based phone) alive.
Treat the fiber pigtail with respect: the connector is precision glass. Don't kink it, don't unplug it out of curiosity, and never stare into the end — the laser light is infrared and invisible.
Reading the lights
Names vary, but the pattern is universal: a power light (should be solid), a PON/fiber light showing the optical link to the network (solid = good), and often a LOS light that turns red when the light signal is lost — a red LOS means the fiber path is broken somewhere and it's time to call the provider, not reboot the router. An Ethernet/LAN light just confirms your router is attached. Thirty seconds of light-reading saves most support calls.