Definition
Indexing — Indexing is the process in which a search engine analyses a crawled page and stores it in its index, from which it can be shown in results.
Quick answer
A page can be crawled but not indexed. Check the Pages report in Search Console: “Discovered” and “Crawled – currently not indexed” have different causes.
The stages
Discovery → crawling → rendering → indexing → ranking. Problems can occur at any stage, and correct diagnosis starts with the question: at which stage does the page stop?
Sources
- Google Search Central documentation — Google