Crawl budget is the number of pages Googlebot will crawl on your website within a given period. Google does not crawl every page of every site constantly — it allocates a certain amount of crawling attention based on how large, important, and fresh it judges your site to be.
Two things shape it: crawl capacity (how much Google can crawl without overloading your server) and crawl demand (how much Google wants to crawl, based on your site's popularity and how often content changes). Together they set the practical ceiling on how much of your site Google visits.
