robots.txt
robots.txt tells search bots which parts of a site they may crawl. One wrong Disallow line can hide a whole site from Google, so it is worth checking after every change.
XML sitemap
A sitemap lists the pages you want indexed. It should be valid XML, contain fewer than 50,000 URLs per file, use your own domain, and ideally include lastmod dates. List it in robots.txt with a Sitemap line.