Impression share
The share of the impressions an ad was eligible for that it actually received, with the remainder split between budget and rank limitations.
Impression share = impressions received / impressions eligible
What makes it wrong.
The mechanism, not the warning.
Eligibility is estimated by the platform, not counted, so impression share is a modelled figure. It is reliable for direction and unreliable as a precise denominator.
In detail.
The value on its own says how much of the available demand was captured. The two lost-share figures beside it say why the rest was not: lost to budget means the money ran out, lost to rank means the auction was not won.
The distinction decides the action. Budget loss is solved with money, rank loss is not.
Where this is measured.
Impression share comes from Google Ads. The server page lists every tool that returns it, with its parameters.