Indexation is the act of Google adding a page to its searchable index after crawling it. A page can be crawled but not indexed (Google saw it and decided it wasn’t worth including), which is a common and often overlooked SEO problem.
Check indexation status per URL in Google Search Console. Common reasons for non-indexation: thin content, duplicate content, noindex tag, canonical to another URL, or low perceived quality relative to competing pages on the same query.