Impression share is a metric that represents the percentage of impressions your AdWords ad received out of the total available impressions in the market you're targeting. The Impression Share report, available through the Report Center, gives you insight into how you stack up against others in your industry, helping you make more effective changes to your campaigns and boost traffic. 

Once you have an understanding of where you're missing out on potential traffic, the next step is improving your ads' impression share. So, before you start restructuring your whole account to increase traffic, get clued in by running an Impression Share report.

Source:→ AdWords Blog