All that is really happening is that you are on a web page that is designed to trick you into calling a phone support number, where support "technicians" will charge you an exorbitant fee to fix your computer. Any by "fix" they really just mean installing more malware and ransomware, further compromising the machine.
You probably got there by clicking a bad link, or misspelling a URL. You'll see a page that looks like this:
Safari - Alert
Your Apple Device has been locked, due to security reasons. You are advised to call the number the helpline number +1 800-870-3001 as soon as possible.
Please do not use your device it may lead to stealing of data, contacts and personal information.
Kindly speak to the customer care representative in order to get this resolved. Call Support for Apple now on +1 800-870-3001.
Your Apple Device has been locked, due to security reasons. You are advised to call the num
It sounds scary, and it will seem like your machine is locked, but it isn't. And there is a very simple fix for this issue: