what is a root kit

It's a malicious software that enables an unauthorized user (hacker) to gain control of a computer system without being detected.

Hackers use rootkits to hide malware on your PC. Malware hidden by rootkits often monitor, filter, and steal your data or abuse your computer’s resources, such as using your PC for bitcoin mining.

By using a rootkit, a hacker hopes to protect and maintain their hidden presence on your PC for as long as possible.

A successful rootkit can potentially remain in place for years if it is undetected. All this time it will steal information and resources from your PC.

Put simply, some of the things your PC does are intercepted by the rootkit.

This means that after a rootkit is installed, you can’t trust any information that your PC reports about itself.

For example, if you were to ask your PC to list all of the programs that are running, the rootkit might stealthily remove any programs it doesn’t want you to know about.

In other words, rootkits are all about hiding things. They want to hide themselves on your PC, and they want to hide malicious activity on your PC.

Fortunately, WSA blocks, detects and removes rootkits. It has a separate shield called the 'Rootkit Shield' for the purpose.

Hello and Welcome to the Webroot Community!
 
Have a look here: http://en.wikipedia.org/wiki/Rootkit and WSA does protect you from Rootkits!
 
Thanks,
 
Daniel 😉

The key different between a rootkit and any other malware is that it infects the machine at the OS level, in some cases before the OS itself loads.  This means that the infection is often invisible to the OS or any antivirus software, and makes removal more difficult.