Disavow
Disavowing is telling Google to ignore specific backlinks or domains when assessing your site, via a file uploaded in the disavow tool.
The disavow tool lets you submit a text file of URLs or domains (formatted as domain:example.com) that you want Google to treat as if they carry no link equity. It was built primarily for sites recovering from an unnatural-links manual action, where you must demonstrate a good-faith effort to clean up links you cannot remove manually.
It matters because used correctly it is a safety valve, and used carelessly it is a foot-gun. Disavowing is irreversible in effect for as long as the file stands, and a clumsy file can strip away legitimate links that were actually helping you. Google's own guidance is blunt: most sites never need it, because the algorithm already discounts spam without your input.
Practical guidance: only reach for disavow if you have a manual action citing inbound links, or strong evidence of a deliberate negative seo attack or your own past link buying. Disavow at the domain level rather than individual URLs, document why, and resist the urge to add every link with a high spam score. When in doubt, leaving the file empty is usually the safer choice.