There have been volumes written, and countless discourses given, to explain exactly what this kingdom is, and it is a deep theological concept. But for Christ to have placed such importance upon it, it must have a certain simplicity to it. So to put it in simplest terms, the “Kingdom of God” is wherever God’s Divine Will is done. The Kingdom of which Christ spoke is both here-and-now as well as something in the future (“thy kingdom come, thy will be done on earth as it is in heaven...”).
Whatever else might be said, the kingdom is in the heart of man, where the Divine Will is discerned and acted upon.