What is "htaccess"
The archive .htaccess is a configuration file for the Apache web server, allowing configuration settings to be made at a specific level of the directory, replacing the entire server settings or at the site level of the main Apache server level configuration. It must be enabled in server level configuration to work.
The most specific settings for directory (though not all) can be configured in .htaccess. One of the most common uses for this is URL redirection using mod_rewrite, which is used by many websites for SEO purposes and to provide easier ways to read Urls. The Apache documentation also has more information on mod_rewrite, as well as simpler solutions for many common cases.
Archives .htaccess can also be used to specify custom error pages (for example, for 404 errors); security authentication and directory specific user, configuring PHP installation; file handlers and MIME types, output compression, and more.
For more information about the files htaccess. And how to use them, see apache documentation.