Definition
Crawl budget is the number of URLs a search engine is willing and able to crawl on a website within a given time.
Crawl Budget explained
Most small and medium websites do not need to worry about it. It matters for large sites, such as ecommerce stores with filters, where millions of URL combinations can waste crawling on low-value pages.
Reducing duplicate URLs, fixing errors and keeping sitemaps clean helps search engines spend their time on important pages.