Indexing is what happens after crawling: once a search engine has fetched and read a page, it decides whether to store that page in its index, the enormous database of pages a search engine actually draws results from. A page has to be indexed to have any chance of appearing in search results at all; ranking is a separate question that only applies to pages that clear this bar first.
Being crawled doesn't guarantee indexing. A search engine can crawl a page and still leave it out, because the content is judged too thin or duplicative, a noindex tag says not to, or the page simply isn't considered valuable enough relative to everything else competing for index space. Google Search Console's URL Inspection tool reports a page's indexing status directly, along with a reason when a page has been excluded, which makes it the first place to check when a page isn't showing up in search results the way it should be. robots.txt affects crawling specifically, not indexing directly, though a page that's never crawled obviously can't be indexed either, which is why the two get discussed together even though they're distinct steps.
