The short answer is plain: crawlers index pages by following internal links. A crawler starts on a known page, reads the links it can reach, and moves from one page to the next. If a page is not linked from anywhere the crawler can reach, it is far less likely to be found.
I keep that fact close because it changes how I read a site. A page is not only a text file on a server. It is also part of a link path. The crawler sees that path first. The page may be present, but if no internal link points to it, the crawler has no clear route in.
That is why internal links matter so much. They are the main way crawlers discover new pages on a site. They also help the crawler decide which pages sit near the center of the site and which pages seem less important. A home page, a menu link, or a link from a well-linked section gives a crawler an easier path than an isolated page buried in the site.
I think the useful point here is not just that links exist. It is that the crawler depends on them. The crawler does not guess the shape of the site. It follows the connections the site makes visible. In simple terms, internal links are the site’s map for the crawler.
That also means indexing and crawling are not the same thing. Crawling is the act of visiting pages and following links. Indexing is the later step, when the search system stores a page so it can appear in results. A page can be crawled and still not be indexed. That is an important limit. Discovery is not the same as inclusion.
The limit matters in another way too. Internal links are the main route, but not the only possible one. A crawler may also find pages through a sitemap or through links from outside the site. Still, internal links do the daily work. They give structure to the site and help a crawler keep moving.
This is where many people misread the problem. They think a page exists, so the search system must know it. That is not how crawling works. A page needs a path the crawler can follow. A clean internal link structure makes that path easier to see. A page with no internal links may be real, but it is weakly connected to the site’s own web of pages.
I want to be careful here. There is still some uncertainty in any single case. Different crawlers can behave in different ways, and search systems change their rules over time. A publisher may describe a crawler in broad terms, but the exact crawl path, crawl depth, and index choice are often not fully visible from the outside. So the claim should stay plain and limited: crawlers index pages by following internal links, but the full result depends on the crawler and the site.
That limit is worth stating because it keeps the claim honest. Internal links are central, not magical. They help discovery. They do not guarantee indexing. They guide the crawler, but they do not force the search engine to keep every page.
For a researcher, the practical meaning is simple. When a site seems incomplete in search, the first thing I ask is whether the page has a clear internal path. If it does not, the page may be hard for a crawler to find at all. If it does, the page is at least visible to the site’s own structure, which gives it a better chance of being reached and processed.
I like this answer because it is small and exact. It avoids drama. It says what the crawler does, what internal links do, and where the limit sits. That is enough for most search questions of this kind. It is also in line with The Source List: one digital source worth knowing, one search tip, and one honest limitation.