.htaccess file:

".htaccess" is the default name of a configuration file that contains "server directives" (commands known by the server) that tell the server how to behave. One common use for an .htaccess file is to restrict access (password-protection) to specific files or directories on the Internet or intranet, or to specify a particular webpage to be accessed when there the file requested by the browser is not found (error 404).