Donald Trump tweeted:
Just started, tonight, our 7th FBI investigation of Judge Brett Kavanaugh. He will someday be recognized as a truly great Justice of The United States Supreme Court!
What were the previous 6 investigations about?
They were background checks or investigations for employment in sensitive positions. For example, here is the policy for "all employees of district and magistrate judges chambers", I can't find the specific policy for circuit judges but it would be similar. https://www.ned.uscourts.gov/internetDocs/pom/crtplans/transcribe/Background_Checks_and_Investigations_Policy.pdf
Note that there is a difference between a check and an investigation, and there are different levels of investigation.