What is the meaning of the word "Rabb" in the supplication "O Allah, Rabb of this perfect call and the standing prayer"?

The phrase "al-Rabb" (the Lord) is applied to the one who nurtures and reforms something. And the meaning of "Rabb of this perfect call" is its owner and the one deserving to be described by it. The "call" here refers to the words of the Adhan (call to prayer) or the call to monotheism.