Hygiene is the practice of maintaining good health and preventing disease, especially through cleanliness. When it comes to human beings, hygiene can refer to many things including brushing your teeth, flossing, showering, and other good practices.
The habit part of practicing good hygiene is the part that interests practical IT security. Good hygiene in IT would consist of establishing similar routines for updating software and the operating system, ensuring the antivirus and other security software is updating and running properly.
IT hygiene can be carried out by the end user or the administrator of the computers. Working on a larger scale, the administrator might use tools like WSUS to monitor computers in need of Windows Updates or vulnerability scanners to identify computers in need of third-party software updates. These might also detect devices on your network, unsupported operating systems, open ports, disabled firewalls, and other configurations that need attention.
Just like you should be brushing your teeth regularly, good IT hygiene would involve a routine and regular oversight to ensure everything is maintained. Good hygiene prevents disease, good IT hygiene prevents vulnerabilities and malware infections.