It's the latest edition of the "Microsoft Tech Support scam," which hijacks users' computers in order to ransom their devices back. The dialogues provide phone number which purports to connect users with Apple's support team for help. Calling the number only connects customers with phony tech support representatives, who will then pressure the caller into handing over their banking and/or social security information.
While current versions of Chrome and Firefox on Windows give users an option to stop prompts from generating over and over again, Safari on iOS does not. If you're stuck in the endless loop of these "iOS has crashed" popups, though, there is a way to break free, courtesy of Intego (opens in new tab).
How To Escape Fake Crash Scams on iOS
1. Pull up Control Center from the bottom of the screen and select the Airplane Mode button to go offline.
2. Open Settings, and select Safari.
3. Click on "Clear History and Website Data"