It means you have articles or blog content on your website which can rank for the same search results in Google. This may be because you optimized them using similar keyphrases or the content in it covers the same topic. This means your content is devouring each other to rank on Google. This happens mostly for growing websites and your content will be competing against itself. Google will show either one or two results from the same domain for a search query but for a high authority domain, it may get three.
If you cannibalize your keywords you will be competing against yourself in Google. If you have two articles on the same topic, it might be difficult for Google to determine which content will rank the highest for a certain query. Backlinks and CTR are key factors that get diluted due to several posts. This results in both ranking low in search results. You should develop a keyword analysis tool to notify you in case of a repeated keyword or optimized keywords.
Cannibalism may also occur if you optimize posts for focus keywords that are not exactly similar but with the same meaning. It becomes difficult for Google to identify which article is more important although their content may have a different perspective. It is easy to identify whether your site suffers from keyword cannibalism. All you need to search in your site for any word you think has multiple results. You do this by Googling ‘site; domain.com and then ‘suspected keyword’ and you will easily get your answer. You can look at your findings in Google using the same keyword on Google local search result checker.