The configuration file that controls a great deal.
WHAT IT CONTROLS
URL rewriting, including WordPress permalinks Redirects Access restrictions Caching and compression headers Error document handling PHP settings, in some configurations
WHY IT MATTERS
A missing or broken .htaccess causes symptoms that look like something else entirely.
THE COMMON SYMPTOMS
Permalinks returning not found Redirect loops A 500 error on every page Access denied where it should not be
THE 500 ERROR POINT
A syntax error anywhere in .htaccess produces a 500 on the whole site.
If a 500 appeared immediately after editing it, that is the cause.
HOW TO TEST
Rename it temporarily. If the site works, the file is the problem.
Rename it back, then find the error.
BEFORE EDITING IT
Copy it first. Always.
FOR WORDPRESS
Re-saving permalink settings regenerates the WordPress section.
WHAT NOT TO DO
Paste rules from the internet without understanding them.