Introduction to the Google coverage report and interpretation of its data
It also lists the problems faced by Googlebot during crawling and indexing.
Error: The page is not indexed. There can be several explanations for this
Excluded: The page is not indexed, Google has followed the rules of your site such as no-index tag in robots.txt
If there is a sudden increase in errors, you can analyze the data and relate it to the impressions as well.