It's a way of warning you that someone else might have made the same point that you're about to make and that you should reply in a fresh thread.
Prepare your replies in a text editor and copy 'n' paste them to the new text entry box.
Prepare your replies in a text editor and copy 'n' paste them to the new text entry box.
Asking users to type in an external text editor, and then copy/paste isn't a reasonable solution for a message board.