A sitemap is a file, usually xml, that lists the URLs on your site you want engines to know about, sometimes with the date each was last changed. It does not force anything to be crawled or ranked. It is a hint about what exists and what is worth looking at.
It earns its keep on large sites, where a crawler might not reach every page by following links, and on pages that change often, where the last changed date tells an engine to come back. On a small, well linked site it matters less, because the links do the same job.
The rule that catches people out is honesty. A sitemap should list only URLs that return 200, are canonical, and are meant to be indexed. Listing redirects, error pages, noindex pages, or URLs your robots.txt blocks sends mixed signals and wastes crawl budget. A URL that is in your sitemap and disallowed in robots.txt is a straight contradiction.
Topkay reads your sitemap from robots.txt, counts the URLs, and cross checks them against your robots rules for that conflict.