What it actually means
When an engineer installs full fibre at your property, they drill a small hole, feed in a fibre cable, and fit a white box to your wall. That box is the ONT. It takes the pulses of light coming down the fibre and converts them into an Ethernet signal that your router understands. The router then handles your Wi Fi, your IP addresses, and everything else.
The ONT belongs to Openreach, not your provider. It stays on the wall even if you switch broadband companies. It needs mains power, so it has a small power brick plugged in next to it. Most ONTs have three or four indicator lights labelled things like Power, PON, LOS, and LAN. PON should be solid green. LOS should be off. LAN flickers when data is moving. Customers often see the red LOS light and panic, when in reality LOS being off is the healthy state.
At home
What this looks like in the house
You have probably never thought about the ONT until the morning the Wi Fi stops and you start hunting for something to unplug. Knowing which box is which saves a panicky call. If the lights on the ONT look wrong, it is usually a fibre issue Openreach has to fix. If the ONT looks fine but the router is sulking, that is usually a quick reboot. Telling them apart is the difference between a five minute fix and a wasted afternoon.
In business
What this looks like at work
In an office, the ONT is the box behind the front desk that no one notices until the day everything stops. A cleaner unplugs it to vacuum, a builder knocks the cable, the power brick gets kicked under a cabinet. Knowing the ONT is the fibre termination, not the router, helps your team describe the fault accurately when they ring support. That alone can cut a multi hour outage to a single short call.