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The Google index is a list of all the webpages that Google is aware of. It is regularly updated with fresh sites and has hundreds of billions of webpages. A web page must be in the Google Index in order to appear in search results.
A page must first be crawled before it can be indexed. Every website is given a crawl budget by Google. "The number of URLs Googlebot can and wants to crawl" is how the crawl budget is defined. If a website has too many pages in comparison to its crawl budget, Google will not crawl and index all of them.