Indexation
Indexation is the process of a search engine storing a crawled page so it can be returned in results. Crawling and indexing are separate steps. A page can be crawled and then deliberately not indexed, in which case it cannot rank at all.
Why it matters for your rankings
Indexation is the pass-fail gate before any other SEO work matters. Keyword research, content quality and link building are all irrelevant for a page Google has decided not to store.
The most common causes are self-inflicted and silent: a noindex tag left over from staging, a robots.txt rule blocking a whole directory, a canonical pointing elsewhere, or thin content Google judged not worth keeping. None of these produce an error anyone notices. The pattern to watch for is 'Crawled - currently not indexed', which is Google saying it saw the page and chose to pass. That is a quality judgement, not a technical fault, and it needs a content answer rather than a technical one.
How to check it on your site
Read the Pages report
In Search Console, Indexing then Pages lists every exclusion reason with example URLs. Start here always.
Inspect individual URLs
URL Inspection shows whether a specific page is indexed, when it was last crawled, and which canonical Google chose.
Check for accidental blocks
Search your codebase and CMS for noindex meta tags and X-Robots-Tag headers, and review robots.txt line by line.
Compare sitemap against indexed count
A large gap between submitted and indexed URLs points at either quality or technical exclusion.