A URL passes three gates: discovery (the engine learns it exists), crawling (it fetches the page), and indexing (it judges the page worth storing). Modern Google routinely declines the third gate for thin, duplicative, or low-authority content — reported in Search Console as 'Crawled – currently not indexed' or stalled at 'Discovered – currently not indexed'.
Fixes are quality-side: consolidate thin pages into fewer substantial ones, add internal links from valued pages, earn external links, and request indexing after real improvements. Noindex-ing genuinely weak pages concentrates the site's quality signal on the rest.