A notice is a message sent to a user or a channel which does not open a separate PM window. In most clients, the notice will appear in whichever active window the user has at the time the notice was sent. In irssi, the notice will appear in the status window by default, but this behavior can be changed. Note that the official IRC specification mandates that no automated replies by sent in response to a notice.
To send a notice to nick1, you can use the following command:
/notice nick1 Message to send to nick1 here
You can also issue a notice to a channel. Note that this is generally considered to be in poor taste and will often be met by a channel ban unless you are a half operator in the channel or above. You must also be in the channel to issue the notice unless the channel is set to mode -n. In practice, that should never happen. To send a notice to #channel, you can use the following command
/notice #channel Message to send to the channel here.