A hosts file is a 'directory' of hostnames and their IP addresses that is used by the operating system to easily find resources, such as other computers or websites.
Maliciously modified hosts file
Modifying and redirecting hosts file entries is a common tactic used by malware to silently send users to an unwanted website. Often, this is used to expose the user's machine to web-based attacks, as well as exposing the user to unsolicited and/or malicious content.
In other cases, the modification is made to prevent the user from accessing certain sites, such as operating system or security software vendors.
Intentionally modified hosts file
Under certain circumstances, hosts files may be manually modified by system administrators or developers in the course of their work.
Such intentional modifications will still trigger the 'redirected hosts file' detection. In such cases, to preserve the modification, the user should exclude the hosts file from scanning once the product detects the change (see section Removal: Exclude an intentionally modified hosts file).