Crawl budget is roughly how many of your pages a search engine will fetch within a window of time. For most small sites it's a non-issue, but on large sites, wasting it on duplicate URLs, endless parameters, or thin pages can mean important pages get crawled less often.
Clean site structure, correct canonicals, a good sitemap, and pruning low-value URLs all help a search engine spend its crawl budget on the pages that matter.
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