Google collects information about security incidents related to websites and eventually reports when they may have been hacked. If you have received a warning, there are signs that a criminal hacker may have altered some of the website’s existing pages or added new spam pages. Visitors to the site may be redirected to unwanted advertising or malware. Check whether there are any signs of an attack by cyber criminals.
If the suspicion is unfounded, follow the instructions from Google: support.google.com/websearch/answer/190597
