Understanding it
Google describes 3 stages: crawling, where a bot discovers and downloads the page, indexing, where it analyzes the text, images and tags, then serving, where it chooses which pages to show for a query.
Google does not index everything it crawls. A page judged too similar to another, too thin, or flagged by the site itself as secondary (a canonical tag pointing elsewhere, a noindex tag) can stay out.
The “Pages” report in Search Console lists indexed URLs and the reasons why others are not. The URL Inspection tool checks a specific page and lets you request indexing.
A concrete example
Illustrative example: invisible product pages
An online store notices that its new products never show up on Google. Search Console lists them as “Discovered, currently not indexed”: Google knows the URLs but has not kept them.
The analysis reveals product pages with text copied from the supplier, reachable only through the internal search. The store writes its own descriptions and links each product from its category. The pages now have unique content and a path of links to be discovered.
The common mistake
Blocking a page in robots.txt to deindex it
robots.txt prevents crawling, not indexing. Google can keep a blocked URL in its results if other pages link to it, without being able to read the tag asking for it to be excluded. To remove a page from the index, use a noindex tag and leave it crawlable.