When Apache Directory Index is enabled, if you browse to a url that have no index file present, you will see list of all files. This is not good for securiy as hackers can see all files present in the directory.
Apache
Apache is a free Web server, that according to a Netcraft Web server survey, powers more than 60% of all Web sites on the Internet. Apache runs on most operating systems including Linux, Windows, Solaris, Digital UNIX, AmigaOS, etc.
www.httpd.apache.org