Snippet competes for the click
Whether this page's search listing gives a searcher a reason to pick it over the results around it.
Ranking is only half the job; the snippet - title and description as they appear in results - competes for the click against everything on the page around it. A generic or uninformative snippet loses to specific, benefit-stating neighbours even from a higher position.
When this is flagged
The report flags this when, read next to the descriptions of the results it competes with, this page's snippet is generic or uninformative - a searcher scanning the results has no concrete reason to choose it.
How to fix it
Write the as one or two sentences of the page's most compelling specifics - numbers, outcomes, what makes it different - in the searcher's own vocabulary. Read it beside the competitors' snippets and ask honestly which one you would click.