I think what you really need to do is study a little more. You're missing some of the obvious stuff. I don't really understand what you're trying to script - can you maybe give a little more information and maybe you could get some more help. I'd recommend you try to download the Scripting Doc from the scripts database.
Regarding your query about the brackets, they contain the actual data extracted from your function (in this case OnEvent_ChatWndSendMessage)
[string] OnEvent_ChatWndSendMessage( [object] ChatWnd, [string] Message );
OnEvent_ChatWndSendMessage - This is your function - will be called when you send a message.
ChatWnd - This will be the chat window ID - this is unique for each window, and so refers specifically to the window which called the event.
Message - This is simply the message you sent.
So, if your intention is to pretend to be your contact for every message you send the script would be similar to this:
function OnEvent_ChatWndSendMessage(ChatWnd, SentMessage)
{
ChatWnd.SendMessage("/usay " + SentMessage)
}
(For the better scripters out there... please feel free to explain better - or more importantly correct me if I'm wrong - I'm really not too sure about any of this myself
)