Impression share is the percentage of potential searches a brand appears on the keywords they are targeting. When looking at a competitor, it is the percentage of potential searches the competing brand appeared on that we are also targeting.
Overlap rate is the percent of times X brand appears on those keywords at the same time as our brand.
Abs top of page rate is how often a brand or ad appears in position 1.
Example: If competitor A has a 16% impression share, it means they appeared on 16% of the same searches we are targeting. If their overlap rate is 23%, it means of that 16% of the time their ad appeared (impression share), 23% of the time our ad was also live. If they have a 28% absolute top-of-page rate, they are in position 1 28% of the time they appear.
This allows an advertiser to understand how aggressive (or not) the competition is in terms of number of Keywords (impressions share) and budget or bids (abs top of page rate)