Can someone explain crawl budget?

How many URLs Googlebot is willing and able to crawl on your site in a given period. It depends on how fast your server responds and how much Google thinks your pages are worth recrawling. Honest answer though, for most sites it doesn't matter. With a few thousand pages it's not your problem. It becomes real on big ecommerce or classifieds sites where filters and parameters create millions of URLs and Google wastes its time on junk instead of the pages you care about.