Keyword cannibalisation is an SEO term used to describe when more than one page on a site is targeting the exact same query. For example, if an eCommerce site sells furniture, having a page for “Shop Kitchen Furniture” and “Kitchen Furniture Items” could be cannibalisation.
This causes a problem regarding SEO because now search engines have to figure out which one to index for “kitchen furniture”. Either they pick one, and the other sees no traffic, or they pick both and split the traffic… and the page authority. Both are likely to see drops in conversion rates.