The Disavow Tool is an instrument provided by Google that allows webmasters to declare certain links to their website invalid. Originally introduced in response to the Penguin update, it was intended to help website owners distance themselves from harmful backlinks, especially if these were bought by the webmasters themselves or their SEOs. However, Google warns that this tool should only be used in certain cases, such as manual spam due to unnatural links. John Mueller from Google has confirmed that the link disavowal process can take months to work its way through Google's algorithm (Source).
Mueller has also criticized SEO companies that offer negative SEO services and link disavow services outside of the tool's intended purpose, as they capitalize on customer ignorance(Source). He emphasizes that many of these practices are irrelevant and that the agencies offering or advising against such services are often making things up (Source).