What is the intended meaning of the Hadith: "Look if my servant has any voluntary prayers, for with voluntary prayers, what was deficient in the obligatory prayers will be completed"? Does it refer to the obligatory prayer itself or to humility in prayer?

The deficiency mentioned in the Hadith is a matter of scholarly disagreement. Ibn al-Arabi and Al-Sindi favored the interpretation that the intended meaning is a deficiency in reward and recompense. They cited as evidence the Hadith: "Then zakat is likewise, then deeds are taken according to that." So, just as zakat is perfected by its excellence, so too is prayer.