It means you don't have the correct codecs. Get codecs here: http://www.land.ee/d_nimo_codec.html OR else here: http://www.click-now.net/html/K-LiteCodecPack.htm

Else get a backup player like DivX player or VLC which has built-incodecs & is free: http://www.videolan.org/vlc/ (choose nearest mirror site)