Your iOS device (Android does not use UDID) has an Unique Device Identifier, called UDID. This is a sequence of 40 letters and numbers which is specific to your device.
All UDIDs need to be in the application file to be able to install it. The developer will insert your UDID in the application for you.
When adding your iOS device to Beta Family, by installing the profile sent to your mail, the UDID will automatically be saved to your profile. If you would like to add it manually, visit https://betafamily.com/superudid from your device.