what need this error?

[edi67]

2: session_start(): Cannot send session cookie - headers already sent by (output started at /home/forumzo7/public_html/Sources/Subs.php:559)
/home/forumzo7/public_html/Sources/Load.php
1159

what need this error ? is first time i see it

[[Unknown]]

This error is caused by another error, which cannot be deduced from the error alone.

-[Unknown]