I had a lot of these pop ups and I don't know if it's for real or not.
Solution: Correct your date, or Set the clock to update automatically
Reason: This security warning pops-up when the verification of certificate needs to be done but the requesting system is set to wrong date(usually to the past).
The security certificate is seen as From Future.
This is all related to certificate validation by the browser.
Related Stack Exchange post
NB: This does not harm your phone (and certainly it's not a virus)
EDIT: There can be other reasons as suggested by @Lie Ryan - due to certificate mismatch / weak encryption / browser not trusting the CA