Three stages, and problems can occur at any of them.
CRAWLING
Search engines follow links and read sitemaps to find pages. A page nobody links to and no sitemap mentions may never be found.
Crawling consumes your server resources. A large site with slow pages may not be crawled fully.
INDEXING
Having crawled a page, the engine decides whether to store it. Thin content, duplicate content, or pages blocked by directives may be crawled and not indexed.
A page can be crawled but not indexed. This is the stage people most often misunderstand.
RANKING
For a given search, the engine orders indexed pages by relevance, quality and many other signals.
WHY THIS MATTERS
If your page is not ranking, establish which stage failed. Not crawled, not indexed, or indexed but ranking poorly are three different problems with three different fixes.
Search Console tells you which applies, which is why it is the first tool to set up.
WHAT YOU CONTROL
Whether pages can be crawled, whether they are worth indexing, and whether they deserve to rank.
