The .htaccess file is a powerful preprocessing configuration system for the Apache web server. It is a simple text file containing sometimes obfuscated regular expression syntax (which although can get obfuscated, is very powerful and terse).

An .htaccess file is usually hidden on FTP,  so in some FTP clients one must enable "show hidden files".

Inside the .htaccess file one can setup a plethora of directives and preprocessing hacks.

More information is available at the Apache website in the documentation.