Googlebot
Google’s crawler. It fetches your pages; a separate system then decides whether to index them.
Googlebot is Google's web crawler, the program that fetches your pages so they can be indexed. It renders JavaScript, but on a delay and not always completely, so anything essential should exist in the server-rendered HTML. What Googlebot can reach is bounded by robots.txt, your server's responses and your internal links.
Definition
Googlebot is the crawler Google uses to discover and fetch pages for its index. It is really two agents sharing a name: Googlebot Smartphone, which does the overwhelming majority of crawling since mobile-first indexing became the default, and Googlebot Desktop, which now sees comparatively little use.
Crawling and indexing are separate steps. Googlebot fetches the HTML, queues the page for rendering if it depends on JavaScript, and only then does the indexing system decide whether the page is worth storing. A page can be crawled and never indexed — that is what "Crawled – currently not indexed" means in Search Console.
Googlebot identifies itself in the user-agent string and comes from published Google IP ranges. Anything claiming to be Googlebot from another range is a scraper wearing its name, which is why reverse-DNS verification matters before you grant it special treatment.
Why It Matters
Everything downstream of ranking depends on Googlebot being able to reach and read the page. A robots.txt rule, an accidental noindex, a 5xx under load, or a render that needs JavaScript Googlebot never gets to run — any one of these ends the story before ranking is even considered.
It also has a budget. On a large site Googlebot will not fetch everything every day, so URLs wasted on parameters, duplicates and redirect chains are URLs not spent on pages that matter.
Example
A site blocked /assets/ in robots.txt to "save crawl budget". Googlebot could still fetch the HTML but not the CSS and JavaScript it referenced, so the rendered page looked unstyled and broken. Google judged the mobile experience on that broken render. Removing the block fixed it — layout and script files must stay crawlable.
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