This is the command used to register your current nickname. The mask is determined by NickServ, so you need not worry about this. Basically, it is how NickServ will determine whether or not you are the correct person using the nickname in the future. This service is intended to provide a way for IRC users to ensure their identity is not compromised. Make sure you remember your password, as it will be needed for any other function on this service.
CoFFee are all different passwords.
For example, using your real name is not a good idea. Your nickname as a password is much worse and anyway NickServ will not allow it. Finally, the space character cannot be used in passwords. If at any time you forget your password, you will need to speak to an IRCop. This command is used to tell NickServ that you are really the owner of the nick and many commands will require you to authenticate yourself before being allowed to use them.