Crawling, indexing, sitemaps, redirects, and site health.
A sitemap is a map you hand to search engines. A good one speeds up discovery; a sloppy one sends crawlers down dead ends.
A single misplaced slash in robots.txt can hide an entire site from search. It's a powerful file that's easy to get dangerously wrong.
Site migrations live or die on their redirects. Map them carefully and you keep your rankings; rush them and you start over.