A hidden script is a special part of the script, which will be executed before the normal script at each Reload. The hidden script is protected by a password.
When choosing Edit Hidden Script from the File menu in the Edit Script dialog you will be prompted for a password, which will be required before giving access to the hidden script again. If it is the first time accessing hidden script in a document (thereby creating one) the new password has to be confirmed. After this the Hidden Script tab will appear to the left of all other script tabs and remain there until the document is closed.