Keyword cannibalization is a Search Engine Optimization or SEO issue that appears when more than 1 page on a site targets the same keywords and gives the same purpose and does a kind of harm for each other's search engine rankings. 
The main reason this happens is because the search engine not able to understand which page has the most relevant result for asked or associated queries.

Let's understand this with an example, let's say we have two different pages about "app development services". If we could get more traffic (organic) by two pages combination into one, then this is called a keyword cannibalization issue on website. 

Is keyword cannibalization bad for SEO?
Yes, it is, but you need to understand that this is important that the real cannibalization issue appears when more than 1 page target the same keyword and harms the organic performance of each other. 

How to find keyword cannibalization issues
Do a content audit to find the issue of keyword cannibalization, you can search the ranking keywords and put your website on search and find, if there are multiple search listings is available then this is the time to fix this issue and try to use other keyword on the low ranking page. 

Run a site search to resolve the issue.
Head to Google and search for site:wemonde.com "app development services". You’ll see all the pages on your site related to that topic.

Hope you are now able to understand that what is keyword cannibalization, please leave a comment below for more information.