An .htaccess file is a configuration file that lives in a website's directory and applies server-level rules to whatever folder it sits in and everything beneath it, without touching the server's global configuration. It works specifically with Apache servers; Nginx handles equivalent rules differently, directly in its own config files.
Common uses include setting up 301 redirects, rewriting URLs into cleaner formats, blocking specific IP addresses or user agents, password-protecting a directory, and forcing HTTPS. For SEO specifically, .htaccess is often where redirect logic lives during a site migration or URL restructure, which makes a small mistake in it unusually consequential: a malformed rule can redirect an entire section of a site incorrectly, or take it down outright, since the file is read on every request to the directory it affects. It's worth editing carefully, testing changes on a small scale before applying them broadly, and keeping a backup of the working version before making changes.
