Cardinality
The number of distinct values a dimension can take, which decides whether its values are reported individually or collapsed.
What makes it wrong.
The mechanism, not the warning.
The catch-all row is easy to filter out as noise, which removes a share of the total without any warning. On a high-cardinality dimension it can hold more volume than the named rows above it.
In detail.
High-cardinality dimensions, page paths or search terms for instance, exceed the number of rows a report can hold. Values beyond the limit are grouped into a single catch-all row rather than dropped.
The grouped row is real data, but it is unusable for anything except knowing how much was grouped.
Where this is measured.
Cardinality comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.