It's program than will scan your computer for Rootkits and attempt to remove them.
Rootkit installation can be automated, or an attacker can install it once they've obtained root or Administrator access. Obtaining this access is a result of direct attack on a system (i.e.), exploiting a known vulnerability (such as privilege escalation) or a password (obtained by cracking or social engineering). Manual removal of a rootkit is often too difficult for a typical computer user, but a number of security-software vendors offer tools to automatically detect and remove some rootkits, typically as part of an antivirus suite. Rootkit detection is difficult because a rootkit may be able to subvert the software that is intended to find it. Detection methods include using an alternative and trusted operating system, behavioral-based methods, signature scanning, difference scanning, and memory dump analysis. Manual removal of a rootkit is often too difficult for a typical computer user, but a number of security-software vendors offer tools to automatically detect and remove some rootkits, typically as part of an antivirus suite. RootkitRemover. McAfee Rootkit Remover is a stand-alone utility used to detect and remove complex rootkits and associated malware. Currently it can detect and remove ZeroAccess, Necurs and TDSS family of rootkits. McAfee Labs plans to add coverage for more rootkit families in future versions of the tool McAfee Labs RootkitRemover. Rootkit Remover is a stand-alone utility used to detect and remove complex rootkits and associated malware. Currently it can detect and remove ZeroAccess, Necurs and TDSS family of rootkits. McAfee Labs plans to add coverage for more rootkit families in future versions of the tool.  Click on image to enlarge. The Rootkit Scan is a tool that checks the file system, the registry and process related lists. When Rootkit Scan is started, it performs a quick scan of a few critical locations. This check takes about a second on modern machines. To check the full system, it is possible to choose a Deep Scan.  BleepingComputer Review: Panda Anti-Rootkit is a free rootkit scanner from Panda Security. This program will scan your computer for Rootkits and attempt to remove them. Unfortunately, when testing this program against two common Rootkits, TDSS and ZeroAccess, Panda Anti-Rootkit were unable to find any traces of it. The term rootkit is a concatenation of root (the traditional name of the privileged account on Unix-like operating systems) and the word kit (which refers to the software components that implement the tool). Manual removal of a rootkit is often too difficult for a typical computer user, but a number of security-software vendors offer tools to automatically detect and remove some rootkits, typically as part of an antivirus suite. Rootkits. Malware authors 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. 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. Our free Virus Removal Tool scans, detects, and removes any rootkit that is hidden on your computer using advanced rootkit detection technology. Rootkits can lie hidden on computers and remain undetected by antivirus software. Rootkit scanning, detection, and removal. 
https://en.wikipedia.org/wiki/Rootkit https://en.wikipedia.org/wiki/Rootkit http://www.mcafee.com/us/downloads/free-tools/rootkitremover.aspx http://www.mcafee.com/us/downloads/free-tools/how-to-use-rootkitremover.aspx https://www.safer-networking.org/scans-for-and-fixes-rootkits/ http://www.bleepingcomputer.com/download/panda-anti-rootkit/ https://en.wikipedia.org/wiki/Rootkit http://www.microsoft.com/security/portal/mmpc/threat/rootkits.aspx http://www.sophos.com/products/free-tools/sophos-anti-rootkit.html
what is rootkit scan