In more detail
Google's explanation of how Search works describes three stages: crawling, indexing and serving results. In the crawling stage, Google finds URLs by following links from known pages, reading sitemaps submitted by site owners and other sources, then downloads the text, images and code.
Google's main crawler is Googlebot, which crawls primarily as a smartphone user agent. Google also renders pages, running JavaScript in a recent version of Chrome, because many sites rely on scripts to show content.
Several things can stop or slow crawling: rules in robots.txt, pages that need a login, links that aren't real HTML links, server errors, very slow responses and URLs that only appear after user interaction. Google's guide to crawlable links explains that it can reliably follow <a> elements with an href attribute.
Crawling isn't instant or guaranteed. Google decides how often to revisit each page, and how much to crawl a site at all. For large sites this is called crawl budget.
Why it matters when hiring an agency
Crawlability is the foundation of everything else. If key pages can't be crawled, content and links can't help them. A good SEO audit starts by checking that search engines can reach every important page through normal links, and that nothing important is blocked.
Ask an agency which crawling tool it uses and whether it will check server logs for large sites. For a quick check on a single page, our Google Index Checker tests whether Googlebot is allowed to fetch it under robots.txt and whether anything blocks indexing.