From what I've been able to gather, it's just a way to reference a script attached to the quest itself, in the scripts tab.
And if you have multiple scripts attached to your quest, you can set it to point to whichever script you choose by quest stage.
Open a quest in the CK and select the "Quest Stages" tab. Select a stage. If there's no stages, create an empty one. Create an empty "Log Entry" if there isn't one of them either.
You can now select what script kMyQuest points to in the papyrus section via the drop down box