Crawl budget is the volume of pages a search engine's crawler will visit on your site within a given time window. Google's crawler doesn't visit every URL on every site; it allocates effort based on site importance + URL value signals.
For large ecommerce sites, crawl budget gets wasted on faceted-navigation URLs (filter combinations that show essentially the same content). The fix: robots.txt disallow on filter URLs + canonical pointing to clean category URLs + noindex on URL variants Google might still discover.
For small content sites, crawl budget is rarely the constraint; quality + linking are. See our technical SEO architecture for the crawl budget discipline.