What it actually means
Installation covers everything between you placing the order and the broadband actually working at the wall socket. The shape of it depends entirely on the technology. An FTTP install almost always needs an Openreach engineer to either run a fresh fibre cable into the property or activate an ONT (the small white box on the wall) that's already there. Once that box is live, you connect the router and you're online.
For FTTC and SoGEA upgrades that reuse the existing copper line into the house, the change often happens at the cabinet end of the line. That can be a self-install where a new router gets posted out and the line switches over on a set date. For 4G or 5G home broadband, the router arrives in a box and you plug it in. Lead times vary widely. Simple line activations can finish in a few days, full FTTP installs are commonly 2 to 4 weeks, and a leased line can run anywhere from 30 to 90 working days because Openreach often has to dig.
At home
What this looks like in the house
Installation is the bit nobody enjoys. The engineer slot that gets booked for next Tuesday morning, then gets pushed to Thursday afternoon, then the engineer doesn't turn up at all. The family stuck on patchy mobile data because the new line was supposed to be live a week ago. Most of the frustration comes from providers quoting an optimistic date to win the sale, then leaving the customer to chase Openreach themselves when it slips. An honest install date is worth more than a fast one.
In business
What this looks like at work
For a business, an install date that slides costs real money. The new shop fitting out for a Saturday opening, then realising on Friday afternoon the card terminal won't have a line. The office move where the leased line was promised for the first of the month, then runs into a wayleave problem and overruns by six weeks. Every day of overrun is staff sitting on tethered phones and customers wondering why the email signature still points at the old address.