Definition
Indexing is the process by which a search engine analyses, understands and adds a crawled page to its index: the database of pages from which search results are assembled. A page that is not indexed cannot rank, however good it is. Crawling (fetching) and indexing (including) are two separate steps; Google crawls far more pages than it indexes.
Between crawling and indexing, Google evaluates whether the page is worthwhile. It looks at the canonical, at noindex instructions, at duplicates elsewhere on the web, at the quality and uniqueness of the content and at the signals of the site as a whole. Pages that are thin, duplicated or barely linked end up in the category 'Crawled - currently not indexed'. That is not an error but a verdict: Google did not consider the page important enough.
You steer indexing with a handful of tools. The robots meta tag with noindex deliberately keeps a page out of the index (for example internal search results or thank-you pages). The canonical designates the preferred version. The XML sitemap tells which URLs you want indexed and when they changed. A request through URL inspection in Search Console or the Indexing API (job postings and livestreams only) speeds up fetching. Robots.txt blocks crawling, but does not prevent indexing if the URL is linked elsewhere.
The 'Pages' report in Search Console is your dashboard. It shows per reason which URLs are not indexed: 'Discovered - currently not indexed' points to crawl budget, 'Crawled - currently not indexed' to quality, 'Duplicate' to canonical problems, and 'Excluded by noindex tag' should only contain pages you deliberately excluded.
AI engines have their own indexes. ChatGPT search and Perplexity maintain an index of fetched pages alongside what the model knows from training; Gemini and AI Overviews draw on Google's index. A page indexed in Google is therefore not automatically known to every engine. Traze submits every published page for indexing immediately, tracks the status in Search Console and checks per engine whether the page is being fetched.