Impression Share
Impression share is the percentage of impressions your ads received out of the total they were eligible for. Google also reports the two reasons you missed the rest: impression share lost to rank (your Ad Rank wasn't high enough) and lost to budget (you ran out of money). It is the clearest measure of how much available demand you're actually capturing.
In depth
The split matters more than the headline number. 'Lost to rank' is fixable with Quality Score and bids; 'lost to budget' is a budget or efficiency decision.
Aim for 85%+ impression share on brand-defense terms, leaving your own brand uncontested is expensive. On broad prospecting, 100% usually means overpaying.
Search lost IS (rank) is one of the most useful diagnostics in the account: it points directly at where better relevance would win more volume without more spend.
Impression share is shown in campaign columns; the related Auction Insights report shows how that share compares to specific competitors.
Common misconception
Low impression share is not automatically a problem to throw budget at. If it's lost to rank, more budget does nothing, you need a higher Quality Score or bid; only 'lost to budget' is solved with money.
Source: Google Ads Help — About impression share
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