The Google Index is the massive database of webpages Google has crawled and stored, from which it draws when generating search results for a given query, representing only a subset of all crawled pages since not everything crawled gets indexed.
A page must be included in the Google Index to have any chance of appearing in search results at all, making index inclusion a fundamental prerequisite before any ranking or optimization efforts can matter.
Checking whether a specific page is in the Google Index, and diagnosing why it might not be, is typically done through the URL Inspection tool in Google Search Console.
Index inclusion monitoring is a standard part of our Technical SEO service.