Keyword cannibalization happens when two or more of your pages target the same keyword or topic. Google can't tell which one to show, so it often ranks none of them well, or keeps switching between them.
Why it matters
Instead of one strong page, you end up with several weak ones that split clicks and links. It's one of the most common reasons a busy blog stops growing.
Example
"How to make cold brew", "Cold brew recipe" and "Best cold brew ratio" are, to Google, mostly the same topic. Three separate posts can end up at positions 14, 22 and 31 instead of one post near the top.
What to do
Group keywords that bring up the same search results into one article. If you already have overlapping pages, merge them into the best one and redirect the others.