What is this PHP warning, “Cannot modify header information”?

[soulcoder]

I'm a newbie for PHP5 and In my php page I'm getting this error when try to redirect to another page\[code\]Warning: Cannot modify header information - headers already sent by (output started at <path to my php file>:<line number>) in <path to my php file> on line <line number>\[/code\]in my php file I have several includes and each one doesnt have any space before of after tagcan anyone help methanks in advancecheerssameera