What It Is
Google finds and ranks pages in three stages. Crawling is Google's software visiting a page and downloading it. Indexing is Google reading that page, working out what it is about, and storing it in its index, the enormous library it searches every time someone types a query. Serving is Google choosing which stored pages to show for a given search. A page has to clear the first two stages before it can compete in the third.
How It Works
Google discovers pages mostly by following links, from other websites and from your own pages, and from the sitemap you can hand it directly. Once it crawls a page, it decides whether to keep it. Google is candid that it does not guarantee to crawl or index every page it finds. It may skip a page that repeats another one, a page with very little on it, or a page carrying a tag that tells it not to index.
Google Search Console is where you can see this happen. Its Page indexing report lists which of your pages are in the index and gives a reason for each one that is not, and its URL Inspection tool checks a single page on demand.
Why It Matters for Being Found
A page that is not indexed cannot rank for anything. It is not competing and losing. It is absent. For a local business this usually shows up in quiet places: the service page for the one job you most want more of, a location page added last spring, a new site whose old pages were never redirected. The fix is often small once someone looks, and Search Console will tell you exactly where to look. Google's AI features draw on the same index, so a page missing from the index is missing there too.