What is the crawl budget?

Crawl budget is simply the frequency with which search engine's crawlers (i.e., spiders and bots) go over the pages of your domain.
That frequency is conceptualized as a tentative balance between Googlebot's attempts to not overcrowd your server and Google's overall desire to crawl your domain.

Crawl budget is the number of pages Google will crawl on your site on any given day. This number varies slightly from day to day, but overall, it's relatively stable.