Google AdWords defines search impression share as "the number of impressions you've received divided by the estimated number of impressions you were eligible to receive." It gets a bit tricky when you factor in searches lost due to budget and rank. This means that your ad could have been displayed to potential customers but was not shown due to budgetary restraints or low ad rank.
Search exact match also determines search impression share. It is derived by dividing your impressions by the estimated number available on the search network for your exact keywords. Understanding search impression share is important to determine the strength of your keywords and lost opportunities due to budget.
Click-through rate is determined by dividing the times an ad was clicked by the times the ad appeared. Specific rates depend on your industry, but if it is extremely high, you have a great ad; if it's extremely low, your ad is lackluster at best. Aabaco Small Business advises using your click-through rate in evaluating ad copy and calls to action and comparing multiple campaigns. While telling you the pool of potential customers, a click-through rate does not tell you what quality that pool is, nor does it indicate your conversion rate. Analyzing your click-through rate helps to determine your cost per click and, eventually, your return on investment.
Your quality score is everything in PPC. Google scores your ad based on the quality and relevance of your keywords. A good quality score will help reduce your cost per click and combine with your maximum bid to determine your ad rank. WordStream lists these algorithm factors used to determine quality score:
Since your quality score determines your budget and, therefore, your exposure to potential converting customers, a higher score generally translates into a lower conversion cost.
Paying for online advertising can quickly become a sinkhole if you are not careful. Paying close attention to metrics like your search impression share, click-through rate, and quality score helps you analyze the efficacy of an ad.
Related "how to" article: How To Improve Lead Generation Using Gated Content
Sources:
Aabaco Small Business - https://www.aabacosmallbusiness.com/advisor/important-click-rate-ctr-really-042603295.html
Wordstream: https://www.wordstream.com/quality-score