Crawl budget is the finite amount of fetching a crawler will do on your site in a given period. It is not unlimited. If a bot spends its budget on the wrong URLs it never reaches the ones that matter.
Budget is wasted on redirects, error pages, pages set to noindex, and duplicate URLs. A single page reachable at four addresses, with and without a trailing slash, with a tracking parameter, over HTTP and HTTPS, is four fetches for one page.
You trim it by keeping internal links pointing at the final canonical URL, stripping tracking parameters, resolving redirect chains, and aligning robots.txt with your sitemap so you are not listing URLs you also block.
Topkay finds this waste in the site audit and the log analyser: the redirects, the errors, and the noindex pages a bot is spending time on.