.htaccess is the name used for files that let you set web server configuration options that apply to the filesystem branch (i.e. resources in the folder and subfolder) containing .htaccess. For more, read Apache's .htaccess tutorial.
Internal Server Error 500
Please note that errors in .htaccess files can commonly cause 500 errors. If using the Rewrite module please ensure you include "RewriteBase /" immediately under "RewriteEngine On" on a new line.