Cannibalisation
Two or more pages of the same site competing for the same query, so neither accumulates the signals a single page would.
What makes it wrong.
The mechanism, not the warning.
It is invisible in any report aggregated by query alone, which is the default view. The two competing pages appear as one line with mediocre performance rather than as two lines that should be merged.
In detail.
Search engines pick one result per site for most queries. When several pages target the same intent, the chosen one varies over time, splitting clicks and making performance look unstable for reasons that have nothing to do with the pages themselves.
It is found by grouping query and page data and looking for queries served by more than one URL over a period.
Where this is measured.
Cannibalisation comes from Search Console. The server page lists every tool that returns it, with its parameters.