The Google index is a process that simply lists all the web pages that Google knows about. It contains hundreds of billions of web pages and is constantly updated with new pages. To appear in search results, a web page must be listed in the Google Index.

To be indexed, a page needs first to be crawled. Google assigns a crawl budget to every website. The crawl budget is simply "the number of URLs Googlebot can and wants to crawl". If a website has too many pages compared to its crawl budget, not all of its pages will be crawled and indexed by Google bots.