Google disavow links help remove hurt backlinks that may be causing your PageRank to be lower then it actually should be.
Google’s PageRank is simply an opinion of what Google thinks your website is ranked out of 10. There are many factors that go into your PageRank but backlinks play a huge part in it, probably more so then a lot of the other factors. In order to get good backlinks or quality backlinks is another article entirely but if you happen to have poor backlinks that either have been automatically or manually generated on another website and you can’t get it off after contacted the webmaster then disavow links is definitely something you’d want to look into. You can access disavow links from your Google’s Webmaster Tools and simply put it allows you to negate any backlinks from any domain name that you enter allowing your PageRank to be ranked more fairly on the backlinks that you do want. You may learn more about Google’s disavow links here.