Average position
In Search Console, the mean ranking of a site's highest-ranking result for a query, averaged over impressions.
What makes it wrong.
The mechanism, not the warning.
Average position improves when a page stops appearing for its worst queries, which looks like a ranking gain and is a loss of impressions. Position always has to be read next to impressions.
In detail.
The value averages positions across every impression, so it mixes results shown at the top of the first page with results shown far down the third. A page ranking first for half its impressions and thirtieth for the other half reports an average of about fifteen, a position it never actually held.
It is a trend signal, not a rank tracker.
Where this is measured.
Average position comes from Search Console. The server page lists every tool that returns it, with its parameters.