This "Household ID" field is only visible if you turn it on. To do so click Admin > Options > Members, then select the last checkbox on the page to "Show the External ID fields on the Member form...".
The purpose of this field is primarily so you can enter a unique identifier in case you want to export the data so it can be used/imported in another database. However, some I think have found a more creative use for it in order to relate a particular household to a security procedure. Perhaps if anyone IS using this field they will share their ideas.
[Note: in the Windows app, the Options tab is under Admin > System Settings. akj]