What it tells you
Impression share is impressions ÷ eligible impressions. If it is 60%, you are missing 40% of the auctions you could have shown in. The value is in the two lost-IS columns: lost to budget means you ran out of money, lost to rank means your Ad Rank was not high enough.
How to act on it
Lost IS to budget on a profitable campaign is a signal to scale spend. Lost IS to rank points at bids, Quality Score, or relevance. Chasing 100% is rarely worth it, since the last slices of impression share are the most expensive and often the least qualified.
Common questions
What is a good impression share?
It depends on goals. For a brand-defense campaign you may want it high; for prospecting, chasing the last points of impression share usually costs more than it returns.
What is the difference between lost IS to budget and to rank?
Lost to budget means your budget ran out before the auctions did. Lost to rank means your Ad Rank was too low to enter or win those auctions. They call for different fixes.