Indexing
Indexing is the stage in which a search engine analyses a page it has crawled and stores it in its index, the database it draws on to answer queries. A page that is not indexed cannot appear in results.
Google describes Search in three stages: crawling, where it downloads pages; indexing, where it analyses their text, images and other content and stores the result in a large database; and serving, where it returns relevant pages for a query. During indexing Google also groups duplicate pages and chooses a canonical URL to represent them.
Google states that indexing is not guaranteed. Not every page it processes will be indexed, and a page can be found and crawled yet still be left out.
Why it matters now: indexing is the gate for AI answers too. Google says a page must be indexed and eligible to show with a snippet to appear as a supporting link in AI Overviews or AI Mode.
Practical note: read Search Console’s page indexing report regularly. Large numbers of pages under “Discovered, currently not indexed” or “Crawled, currently not indexed” usually point to pages Google does not consider worth storing, and adding more of the same will not change that.