An XML sitemap is a file listing the URLs on your site that you want search engines to find, usually at yourbrand.com/sitemap.xml. Google says it helps discovery and does not guarantee that anything in it will be crawled or indexed. A small, well-linked site may not need one; most sites should have one anyway, because it is free and it is how you tell Google what changed.
What Google says it is for
Discovery. It tells crawlers which pages exist, when each last changed, and where alternate language versions live. Google's guidance says you might not need one if your site is small, roughly five hundred pages or fewer, and comprehensively linked so that every important page can be reached by following links from the homepage. Larger sites, new sites with few external links, and sites with pages that are not well linked internally are told to have one.
What it does not do
It does not get pages indexed. A URL in the sitemap that nothing links to and nothing recommends will often sit as discovered, currently not indexed for months. The sitemap announced it; nothing argued for it. Links do that job, and the sitemap is a supplement to them, not a replacement.
It also does not carry priority. Google states that it ignores the priority and change-frequency fields entirely. The last-modified date is used, but only if it is accurate; a site that stamps today's date on every URL on every build teaches Google to disregard it.
How to do it properly
Generate it from the same list of pages the site is built from, so it cannot drift. Include only pages that return a normal status and that you want indexed: no redirects, no noindexed pages, no parameters. Set last-modified only when content actually changes. Reference it from robots.txt and submit it once in Search Console, then leave it alone.
Reading the sitemap report
Search Console shows how many sitemap URLs are indexed and lets you filter the page indexing report to sitemap URLs only. That filter is the quickest way to see which of the pages you care about Google has declined, and the answer is nearly always fixed by links, not by resubmitting the file.
Sources
Where this shows up in the tracker
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Related terms
Discovered, currently not indexed
Discovered, currently not indexed means Google knows a page exists, usually from your sitemap or a link, but has not yet fetched it. The page has not been judged and rejected. It has been queued and left there. On a small site that almost always means Google has not found a reason to prioritise it, not that your server is overloaded.
Internal linking
Internal linking is linking from one page on your website to another. It is how Google discovers most of your pages, how it works out which ones you consider important, and how it learns what each page is about from the words used to link to it. It is also the one ranking lever that costs nothing and that you control completely.
robots.txt
robots.txt is a plain text file at the root of your website telling automated visitors which parts they may request. It is the single highest-consequence file on most websites, because one wrong line can remove you from search results entirely.
Crawl budget
Crawl budget is the set of URLs on your site that Google can and wants to crawl, limited on one side by what your server can handle and on the other by how much Google wants your pages. Google says the concept matters for sites with a million or more pages, or ten thousand or more that change daily. A small business site does not have a crawl budget problem. It may have a crawl demand problem, which is different.