PasswordField help.

See the example on this page:

http://docs.unity3d.com/Documentation/ScriptReference/GUI.PasswordField.html

It shows you how to make a password text entry box.

(If you mean, how does your code know that the correct password has been entered, then the answer depends completely on how secure you need your system to be. If you do not need security, then just compare the response from the PasswordField with the password string. If it needs to be more secure, then use an MD5 hash of the password, and compare that with a hard coded MD5 hash. If you need yet more security, then take a look at this site.)