Crawl Budget
Crawl budget is the number of URLs that Googlebot can and wants to crawl on a website within a given period. It results from the crawl capacity limit (how much time and how many parallel connections the server can offer Googlebot without becoming overloaded) and crawl demand (how relevant and up to date Google considers the content).
In practice
For most small and medium-sized websites, crawl budget is not a problem, because Google crawls them fully anyway. According to Google it becomes relevant above all on very large sites with more than one million unique pages whose content changes weekly, on medium-sized sites with more than 10,000 unique pages whose content changes daily, and on sites with many technically unnecessary URLs, for instance from filter combinations or duplicate content; Google explicitly calls these figures rough guides rather than thresholds. Wasting crawl budget risks new or updated pages being indexed late or not at all. Countermeasures: exclude unnecessary URLs from crawling via robots.txt, consolidate duplicate content instead of merely bundling it with a canonical (both URLs continue to be fetched), improve server response times and avoid redirect chains. Google explicitly advises against noindex as a budget control: the page is requested anyway and the crawling time is lost.