How often an ad was among the ads at the top of the page, or the very first, out of the times Google estimates it was eligible to be.
Google Ads has two columns for the top of the page. Search top IS is the impressions you got among the top ads, divided by the number of top impressions Google estimates you were eligible for. Search abs. top IS is the impressions you got as the very first ad, divided by that same estimate.
Each has its lost columns, Search lost top IS and Search lost abs. top IS, split into (rank) and (budget): the share of the top, or of the first place, missed because the ad's Ad Rank or its budget fell short.
They're easy to confuse with Impr. (Top) % and Impr. (Abs. Top) %, which divide by the impressions you did get. If you show less often, those rates can stay high while the shares fall. Google counts at most one impression per advertiser per search for the rates, and doesn't recommend them as bid targets.
On a brand campaign, Search abs. top IS is often the first column to move when another advertiser bids close to you on your name: you can stay on the page, and among the top ads, and lose only the first place. Read it beside Search lost IS (rank) and the guide to your brand campaign's impression share.
Google's own help and policy pages first.