It is the parent key for saving data. The function is like this:
SaveTriggerHandle(hashtable, parentKey, childKey, trigger)
StringHash() is a separate function, it hashes the given string to form an integer, which is then used as a key. You could just as well use 0, like this:
Custom script: call SaveTriggerHandle(udg_hash, 0, 'AHhb', gg_trg_Untitled_Trigger_071)