Crawling
Also known as: Indexing, Crawl budget
Crawling is the process by which a search engine's bot discovers and downloads web pages; indexing is storing and organising what it found.
In full
Crawling is the process by which a search engine discovers and downloads pages, following links from page to page and consulting sitemaps for URLs it might otherwise miss. Indexing is the separate step of analysing what was downloaded and storing it so it can be retrieved for a query. The two are distinct and often confused: a page can be crawled but not indexed, if the engine judges it duplicative or low value. Crawl budget describes the finite attention an engine allocates to a site, which becomes a real constraint on very large sites and is essentially irrelevant on small ones.
Why it matters
A page that is not indexed cannot rank for anything. Google Search Console and Bing Webmaster Tools both report which of your pages are indexed and, more usefully, why the others are not.
A common misconception
That submitting a sitemap guarantees indexing. A sitemap is a suggestion about what exists, not an instruction about what to store.