Original: The early leaders of the Muslim nation following Muhammad's (570–632) death were called "Khalifat Rasul Allah", means the political successors to the messenger of God (referring to Muhammad).
Simplified: Some of the early leaders of the Muslim community following Muhammad's (570–632) death called themselves "Khalifat Allah", meaning representative of God. But the other title of "Khalifat rasul Allah", meaning the successor to the prophet of God, became the common title.