Google crawling budget - what is that?

Simply put crawling budget is a number of pages Googlebot crawlers are allowed to visit and index within certain time period.

Why is it important for you to know about this?

In case your site exceeds crawling budget you'll have a bunch of pages that will not be indexed until your website grows some age/authority.

It might be interesting to note that you understand that Google (and other search engines) actually have real-life costs included while crawling web properties - in terms of bandwidth spent as a minimum cost as well as processing all of that data.