Hi Lynn,
Survey123 add-ins are currently in development and the item you found is being used for testing and development work in the alpha builds of Survey123 Connect and Field App. There is a new item type of .surveyaddin which will be used in the future to support this new feature. Currently it is not documented and there is no more information available as we have not yet even released this as available in the beta version.
Once the Survey123 beta version is available with this feature, you will be able to find out more information on the Early Adopter Community. Please stay tuned for updates...
Regards.
Phil.