Index
The index is the database where a search engine stores the pages it has crawled, so it can return them as results.
A page only appears in search results if it is indexed. Passing the crawl is not enough: Google then has to decide the page deserves a place in the index.
A page can be left out of the index for several reasons: duplicate content, a noindex tag, low perceived quality. Checking indexing in Google Search Console is part of basic site monitoring.