What are impression share and lost IS (budget vs. rank)?
Impression share is the percentage of eligible auctions your ad actually appeared in; 'lost IS' splits the missing percentage into two separate causes: lost to budget (your campaign ran out of budget before the day ended) and lost to rank (your ad quality/bid combination lost the auction outright). The two require completely different fixes, so treating them as one number leads to the wrong action almost every time.
Search impression share is reported per campaign as a percentage: of all the auctions your ads were eligible to enter, what fraction did you actually show in. The gap from 100% breaks into two mutually exclusive buckets Google reports separately: impression share lost to budget (you were eligible but your daily budget ran out) and impression share lost to rank (you were eligible and had budget, but lost the auction on ad rank).
These two have opposite fixes. Lost-to-budget means the fix is more budget, assuming the campaign is already profitable at the margin, since the demand was there and you simply couldn't afford to capture it. Lost-to-rank means more budget does nothing, the fix is a higher bid, better ad relevance/Quality Score, or accepting you're not going to win that segment of auctions at your current economics. Reading a rank-driven impression-share gap as a budget problem (and raising budget) wastes spend without moving the number that's actually constrained.
One subtlety worth flagging: a rising impression share alongside flat or falling raw impressions isn't necessarily progress. It can mean the campaign's eligible-auction pool itself shrank (a bidding target got more conservative and the campaign is now competing in fewer auctions), which mechanically raises the percentage without the campaign actually winning more of the original opportunity. Always read impression share alongside the absolute impression count and the pool it's a share of, not as a standalone trend.