When you Forward a message you pass a message that has been sent to you on to somebody else.  Forwarding is a bit like replying, except Messenger doe not automatically address the new message. When you are viewing a message you can either click on the Forward button or select Forward from the Message menu. 

When you Forward, Messenger automatically opens a new message and attaches the original message as an attachment. You can compose a message to go with the forwarded message and add the new recipients. 

When you have finished composing your message click on the Send button.