Keyword cannibalization happens when certain pages within your site compete against each other for certain keywords. From the word cannibal, your pages are eating the popularity of other pages to gain rankings. Sometimes it’s an unavoidable occurrence because as we develop page contents, we sometimes have an inherent intention to rank for keywords regardless of which pages they appear. Instead of mapping contents/pages to keywords, we think it’s not so bad to place important keywords on all pages.
It refers to a condition when certain keywords from your own webpages compete with other keywords of your website. It is basically a very minor problem which doesn't pose any significant threat, unless you are under very high competition with cannibalized keywords.