A search engine's index is the enormous database of web pages it has crawled, processed, and stored so they can be retrieved and ranked for a query. The rule is blunt: if a page isn't in the index, it cannot appear in results, no matter how good it is.
Getting indexed is a pipeline, not a single event. A crawler fetches the URL, the page is rendered (JavaScript included), its content and links are parsed, and Google then decides whether the page is worth storing and how to represent it. Each stage can fail: a URL blocked in robots.txt never gets crawled; a thin or duplicate page may be crawled but dropped at the indexing decision.
You can see a page's status in Google Search Console's URL Inspection tool, or approximate coverage with a site:yourdomain.com search. Common reasons a page stays out of the index include "Discovered - currently not indexed" (Google knows about it but hasn't prioritised crawling), stray noindex tags left over from staging, canonical tags pointing elsewhere, and low perceived value.
An e-commerce site launched 4,000 filter-generated URLs and watched only 600 get indexed. The rest sat in "Crawled - currently not indexed" because they were near-duplicates. Consolidating filters, adding canonical tags, and submitting a clean XML sitemap lifted meaningful indexation to 1,900 pages — and those were the ones that actually earned traffic.
Diagnosing and fixing indexation issues sits squarely within technical SEO, and a healthy XML sitemap is one of the simplest tools for guiding it.