This is the name of God that he revealed to his people in the Old Testament. See the translationWord page about Yahweh concerning how to translate this.
The phrase "by the hand of" is an idiom that means that Yahweh used Malachi to deliver his message. AT: "through Malachi" or "spoken to them by Malachi" (See: [[rc://en/ta/man/translate/figs-idiom]])