Without context it is difficult to say much about the meaning of this. You say it showed up on your phone out of nowhere but have given us no description or context for when and where this has occurred.
You opened your phone and after unlocking this message popped up or was there waiting for you?
You were in an app and suddenly this popped up?
Was it a question requiring you OK, Accept or Decline?
How often has this happened since it started happening?
When did you notice this happening?
Does it appear to want to redirect you to a link? Is it trying to get you to protect yourself from something? This seems like the kind of scare tactic anti-virus and anti-malware apps use to get users to upgrade to premium $ervice$ or to buy $omething.
Is it a pop-up or a notification? If it is an notification what app is sending it? You can pull down the notification bar and long press on the notification to see what app is send it.