Karate

Hey, i have a new computer since 3 days and my security scanner is scanning in a full scan a rootkit. It is 99% and at the screen is written rootkit 99% and nothing happens. I did not do anything with the PC, my Firewall was always active and i dont know, why there is malware. My question: What is a Rootkit and what should I do now? I have McAffee. Thank you very much.