Returns 0 if a quest has not yet been completed, 1 if it has.
This function, which is used for condition checks on dialogue, AI packs, Story Manager events, and other similar things is broken as of at least Patch 1.6. It may never have worked though. The function will always return a 0, which renders the check useless until the developers fix it.
You can work around this by using the GetStageDone condition check instead on a specific stage of the quest. Usually the last stage it has.
Note that as of Patch 1.9.32, this appears to have been fixed and will now return the proper values.