Indexing is the step after crawling. The search engine reads the page, works out what it is about and decides whether to store it in its index, which is the database used to answer searches. Only indexed pages can appear in results.
Pages are commonly left out for reasons you can fix: a noindex tag, a canonical URL pointing elsewhere, near-duplicate or very thin content, errors, or simply low perceived value. Being in your XML sitemap is a hint, not a promise.
To check, use the page indexing report in your search console and inspect individual URLs. If important pages are not indexed, look at what the tool says before changing anything. Being indexed is required for search ranking, but it does not decide where you rank.
Related: Technical SEO, Search Engine Optimization, robots.txt.