Password compromise

Without more detail it sounds quite possibly like an alert from Keychain re: weak, reused, or otherwise compromised passwords.

This is a fairly recently added - and quite worthwhile - security tool within iOS.

You can check for this yourself at Settings -> Passwords -> Security Recommendations

What your phone is doing - and how it’s doing it - is detailed in two articles beginning here:

https://support.apple.com/en-au/guide/security/sec7f0432063/web

Need more background. What device? Which system?

Was this a true "notification" of the kind that the system produces in an official box, or were you just "notified" by an email or a Safari pop-up. If the latter two then it is phishing. If it's a system notification, I have never heard of one like that but that doesn't mean they don't exist.

Identifying legitimate emails from the iTunes Store - http://support.apple.com/HT201679 - lacking in specifics, the next link is better:

Recognize and avoid phishing messages, phony support calls, and other scams - https://support.apple.com/HT204759