The Dark Web is a hidden part of the internet used for illicit purposes. Hackers put stolen credentials up there for sale. A password prefix can help to protect you!
Create your password using the first few letters of the system you are logging into followed by a series of characters unique to this specific password. For example, LinkedIn Password: Link73Ark44!
If LinkedIn is compromised, your credentials may end up for sale on the Dark Web but on a Dark Web scan report, the Password Prefix will alert you to change your password on LinkedIn, without worrying about the integrity of other passwords in other systems.