Rootkits can modify the operating system or any other software on the target computer, allowing the adversary to access information, manipulate files, or carry out other malicious activities without being detected. They can operate in kernel mode, allowing them to maintain control over the system while obscuring their presence from regular users and security systems. Rootkits can be installed through various means, including exploiting vulnerabilities, utilizing social engineering tactics or through malware. They are particularly dangerous due to their stealthy nature and the ease with which they can be used to conduct further malicious activities, such as stealing data or launching attacks on other systems.