This will lock your server access to anyone who doesn't have the password. In order to connect to your server with most clients users would have to type the following sequence (assuming port 6667 is enabled for your server) /server your.server.name:6667 thePassword. If they do not but match a later non-password protected entry, then they will be able to login with that client class.
/as client z-default set password bleh
With some clients you will need to use a graphical user interface to log in and they may have a field for the password. If that is the case, try logging in using the interface. Some clients may not be able to handle the command properly; so do not use password protected client classes unless you are sure the intended users can access them. When using the text command, make sure to include the port number. While the port is usually not mandatory, if you exclude it then the password will be in the port field, and the login will fail. Password protected client classes allow you to only allow people who know a particular pass to log into your server. This can be used as a security measure to restrict users to those whom you choose, without having to add a mask for each of them. You can also use it if you wish to allow on any java user, but only allow non-java users that you specifically approve. Or your regular clients can be limited to one login per address, but special exceptions can be given a password to log in with. Another possible use is that the main client class can be limited to fewer users than the server can handle, and a password protected earlier client class can have a large number of allowed users. Then these users will have a better chance of being able to log in if your server tends to fill up. Even if the main client class is full, there will be reserved spots for those with the password. Just remember that to use a client class the user must match both the password and a mask set for the class.