STAC stands for Service Termination Authorisation Code.
In short, if you want to switch to a new provider but aren't bothered about moving your existing number to that new carrier, you can request a STAC code, provide it to the new carrier and your old number will be cancelled.
To switch your services to us and cancel the previous number, you'll need a STAC. To request a STAC code text STAC to 75075.