From a little web searching on this I suspect this is adware - a pop-up that tells you you have a virus, by any chance?
If so, don't click on it - it might install something nasty. It's probably an app you've installed recently which is responsible. You could use an app like Addons Detector or Lookout Ad Network Detector to try to find what app is responsible. Look in particular for apps that use push advertising/push notifications - his type of thing is supposed to be banned from the Play Store, but there may be cases still around.
If you can't identify it, try disabling or uninstalling apps you installed a little while before this started (days or weeks, not hours or minutes). If it stops, you could re-enable them gradually.
Given that aggressive adware is supposed to be banned from the Play Store, apps you've installed from other sources, if any, should be particularly suspect.
Good luck!