A disavow file is a plain text list of URLs or entire domains a site owner submits to Google, asking it to ignore those links when evaluating the site's backlink profile. It's a defensive tool: rather than trying to get a spammy site to remove a link, which often isn't possible, it tells Google directly not to count that link at all.
The most common reason to use one is negative SEO, where a competitor or bad actor points a batch of low-quality or spam links at a site specifically to trigger a penalty, but it's also used after a manual action tied to unnatural links, where Google expects to see cleanup effort before lifting the penalty. It's a blunt tool meant for genuinely harmful links, not a routine maintenance task; disavowing links casually or in bulk without real cause can strip out legitimate link equity a site was actually benefiting from. Google's own guidance treats it as a last resort for links a site truly can't get removed any other way.
