The share of eligible Search impressions a campaign's ads missed because its budget ran short.
Google Ads gives two reasons for the times your ad could have shown and didn't. Search lost IS (rank) is the share missed because the ad's Ad Rank was too low; Search lost IS (budget) is the share missed because the budget wasn't enough. Google gives the budget column at campaign level only, and its API reports anything above 90% as 90.01%.
Together with impression share the two make the whole: shown, lost to budget, lost to rank. A daily budget that runs out by the evening shows up here: the morning's auctions get your ad, and the night's don't. Search lost top IS and Search lost abs. top IS each have a (budget) version too, for the top of the page and the very first place.
On a brand campaign, it should be zero. Every search your ad misses for budget is a search for your own name where the top of the page is open to anyone else eligible, often at the hours your team is least likely to look. If it's above zero, fix the brand campaign's budget before reading anything into another advertiser's share.
Google's own help and policy pages first.