Crawl budget is the number of pages a search engine is willing and able to crawl on a site within a given stretch of time. It's not unlimited: search engines allocate crawling resources based on a mix of how important and how reliable a site appears to be, so a slow or low-authority site simply gets fewer crawl requests than a fast, well-established one.
For most small to mid-sized sites, crawl budget is rarely the bottleneck; there's more than enough to cover every page that matters. It becomes a real constraint on large sites, e-commerce catalogs with tens of thousands of product pages, or sites with a lot of low-value URLs like faceted search results and filtered views eating into the budget that should go toward pages worth indexing. Site speed, a clean internal linking structure, and keeping thin or duplicate pages out of the crawl path (through robots.txt, noindex, or canonical tags) are the main levers for making sure crawl budget gets spent on the pages that actually matter.
