Usage: PASS <password>
The PASS command has a similar functionality as the IDENTIFY command, you can use PASS with your nick password and it will be used when you change your nick to it.
This is a good way to identify for the nick you are using. It requires less typing than the normal method so it is easier and you are less likely to make a typo. It is important to send passwords securely so that you do not accidentally send your password to another user.