He is saying that you have to use the on notice event.
$target is an identifier that returns the target for an event.
If someone talks in #mIRC, it's a privmsg to #mIRC that is sent, the on text event would trigger with $target returning #mIRC.
If someone sends a notice to #mIRC, it's a notice to #mIRC and on notice will trigger with $target set to #mIRC.
If someone sends a notice to @#mIRC, it's what you call a onotice, $target should be @#mIRC.
So inside your on notice event, check if the first character is a @:
if ($left($target,1) == @) { echo -s this is a onotice event }