What it actually means
Number porting is what happens behind the scenes when you switch provider but want to keep the number you already have. For a landline it's a request that travels between your old provider and your new one through Openreach's systems. For a mobile it's handled through the industry's mobile number portability process, usually with a PAC code that you ask your current network for and pass to the new one.
The time it takes varies. A mobile port often completes on the next working day. A landline port can take a few working days, sometimes longer if there's a complication on the line or the address. During the port your number stays live, so calls keep landing on the old service until the switch over moment, then route to the new one. You shouldn't lose calls on the day, but planning the cut over for a quiet window is sensible if the number matters to you.
At home
What this looks like in the house
The phone number has been on the fridge for fifteen years. Grandparents know it, the school knows it, the dentist knows it, and the takeaway has it saved. Changing provider should never mean changing that number, because the felt cost of a new number is every missed call from a friend who tried the old one and got nothing. Porting is what makes a switch feel boring in the best way, the broadband or phone changes, the number on the fridge doesn't.
In business
What this looks like at work
For a business the number on the website, the van, the Google listing and the business cards is a marketing asset. A missed inbound call because the number didn't port cleanly is a sales call walking out of the door. If the number is on stationery you printed two years ago, you can't just change it and update everyone. Porting protects that asset, the till keeps ringing, the booking diary keeps filling, and the customer never knows there was a switch in the background.