In Plain English
Indexing resembles adding information about a book to a library catalogue. After a page is crawled, a search system may process its text, media, links and other signals, then decide whether information about it belongs in an index. A crawled page may not be selected: duplicate, inaccessible or low-value pages can be left out, and a noindex directive can prevent inclusion. Being catalogued only makes retrieval possible; it does not reserve a prominent shelf position.
Why it matters
Only eligible indexed information can normally be retrieved for relevant searches. Separating crawling from indexing helps diagnose the right problem instead of assuming that every reachable URL should appear in results.
Indexing choices can change as pages and search systems change. A page that is absent today is not necessarily suffering from one technical fault, and repeated submission does not improve its underlying value. Check whether the preferred URL is crawlable, returns useful content, avoids an accidental noindex, and sends consistent canonical signals. Then assess whether it adds something distinct enough to retain rather than merely reproducing another page.
Example
A search system crawls two near-identical printer-friendly pages, processes both, and chooses one preferred version for its index.
Common misunderstanding
Indexed does not mean “ranking first” or even appearing for every relevant phrase. Ranking is a later, query-specific decision.