The Disavow tool is a part of Google Search Console that was introduced in October 2012. It enables you to discount the value of a backlink in order to prevent link-based penalties. It also protects the site from bad links that may harm the website’s reputation.
Using this tool, you can tell Google that you don’t want certain links to be considered to rank websites. Some sites who buy links may suffer the penalty if they don’t get these links removed using Disavow tool. The low-quality backlinks, which you don’t control, may harm your site’s ranking. You can ask Google not to consider them when crawling and indexing your site.