Impression Share refers to the percentage of Ad Impressions your Brand received for targeting your are purchasing.
For example, a 78% Search Impression Share means your ads are showing 78% of the time your target audience is searching. This is telling you that there are 22% more opportunities available for you to reach your target market on Search, typically accessible for extra ad spend.
If you have available Impression Share on campaigns that are returning high ROAS, give them more budget. These are low hanging fruit opportunities to feed your winners.