This property contains a list of one or more properties whose values (for security) should not be written into any MSI logs etc. Properties names are separated by semi-colons.
Depending on the logging level Windows Installer may still log the values for a "hidden" property if its value is supplied on a command line.
The PropertyList command has been designed to manipulate this type of property list.
Please see Microsoft's online documentation for the "MsiHiddenProperties" property (or "MSI.CHM") for more information.
You might also wish to have a look at the "Hiding Sensitive Information" section.