Kabbalists believed that there is a gap between how mankind can understand God and what God actually is. This unknowable aspect of God is called Ein-Sof.
The Tetragrammaton refers to the ineffable name for God in the Hebrew Bible. It is forbidden for Jews to say or write the full Tetragrammaton, so they substitute the term Adonai. Christians, however, have no such rule, and in Latin, it is translated as YHWH and is pronounced Yahweh or Jehovah. Read articles on the Tetragrammaton below.