Examples and explanations:
If your e-mail address is firstname.lastname@example.org, then your userid is jsmith.
The password field is your current password.
The reason the form asks for your new password twice is simply to make sure you type it correctly. (You'd be in trouble if you made a typo and couldn't guess what the typo was.)
Changes take effect immediately.
Only the VMail userids and passwords are accessable to this script (e.g. POP accounts).