The Home ID field is the ID number that the Z-Wave Remote Control Unit assigned to your house, so to speak, to differentiate Z-Wave devices in your home from Z-Wave devices in a neighbor's home. (Z-Wave signals can be transmitted up to 100 feet and through walls, ceilings, and floors, so homes with close neighbors would affect devices in a neighbor's homes if each home wasn't uniquely identified.)
This information is read from the controller when you are learning devices. If you already have a Home ID and devices in your HomeSeer interface when you learn devices from a different master controller, the devices and Home ID in the controller will be replaced with the new controller's information.