What it actually means
The thing most people get wrong about ISPs is assuming the broadband itself is different from one to the next. In the UK, almost every ISP is reselling the same physical Openreach line that runs into your wall. The cable is identical. The speed tier is identical. The engineer who fixes it when it breaks is the same engineer regardless of which logo is on your router.
What actually differs is the company sitting between you and that cable. The ISP is the bit you experience every day. They set the price, they write the contract, they decide whether you sit on hold for forty minutes or talk to a human in under a minute, and they decide how hard they chase Openreach when there is a fault. Two ISPs can sell you the exact same connection and feel completely different to live with, which is why the choice of provider matters far more than people realise.
At home
What this looks like in the house
You only really notice your ISP twice. Once when the bill goes up without warning, and once when the broadband stops working before a weekend. The cable is shared with the household down the road. The forty minute hold and the bill creep are not. Picking an ISP is picking the company you want to deal with on those two days, because the rest of the time you forget broadband even exists.
In business
What this looks like at work
For a business, your ISP is who you ring when the office goes dark and the card machines stop. The cable underneath the pavement is the same as your neighbour's. The speed at which someone picks up the phone, raises a fault, and sends you a written update is not. A bad ISP can turn a two hour Openreach repair into a full day of lost trade, because nobody chased anything. The contract you signed decides who carries that risk.