Link disavowal tools are tools offered to webmasters and SEO pros by search engines to prevent bad influence of inorganic backinks on rankings.
By using disavowal tools, webmasters indicate to search engines to ignore low-quality targeted links.
Link disavowal tools are backling cleaning tools.
More on the procedure on Google link disavowal tool.
Below, the tool integrated in GWT:
Matt Cutts explains the "Disavow links" feature of Google Webmaster Tools :