Is done with the use of script, code and automation tools by a QA automation engineer.
Imagine that a programmer updates a website, this happens very often and every time you need to go to the same website/app/program and check the same registration, login and other features over and over again.
How awesome would it be to have all those buttons and links checked automatically so you could grab a coffee meanwhile, right?
So, as a QA automation engineer you write a code which will test all of this for you.