Impression share is one of the most important metrics’ which helps to measure a reach of search network campaign. This particular Google Adwords metrics shows the percentage of impressions where the text ads were shown out of the total possible impressions for targeted keywords.
Lost IS Budget column shows the percentage of impressions were lost because of too low budget.
Lost IS Rank column shows the percentage of impressions were lost because of too low ad rank. The reasons for low ad rank can be various: too low max cpc and low quality scores.
The cheapest way to increase impression share is to improve ad rank whilst increasing max CPC or optimising entire Adwords campaign with an aim to improve quality scores.