Display Impression Share is calculated by dividing the number of impressions received on the Display Network divided by the total estimated number of impressions you were eligible to receive.
The higher the impression share the better.
Display Impression Share can show you how effective our targeting is on the Google Display Network. A low impression share (<10-20%) can suggest that the bids or budget are too low. Best practices for Display targeting is always to use multiple ad groups, a different ad group for different targeting option. Impression share can greatly help when it comes to ad scheduling,