A Google Webmaster Help thread has Google's John Mueller responding to webmaster concern over off topic links.
Off topic links are when sites that are unrelated to you are linking to your site. In the link craze these days, Webmasters are afraid of getting link penalties and thus, they are concerned with off topic links.
Google's John Mueller went on record saying:
Just to be completely clear on this: you do not need to disavow links that are from sites on other topics. This tool is really only meant for situations where there are problematic, unnatural, PageRank-passing links that you can't have removed.
So not all off-topic links are problematic or unnatural.
Forum discussion at Google Webmaster Help.