Question: "What is YHWH? What is the tetragrammaton?"
Answer: The ancient Hebrew language that the Hebrew Scriptures were written in did not have vowels. In the original Hebrew, God’s name is given as "YHWH." This is known as the tetragrammaton. Because of the lack of vowels, Bible scholars debate how the tetragrammaton “YHWH” was pronounced.
Contrary to what some Christians (and at least one cult that uses this name) believe, “Jehovah” is probably not the Divine Name revealed to Israel. Due to the Jewish fear of accidentally taking God’s Name in vain (Leviticus 24:16), they basically quit saying it out-loud altogether. Instead, when reading, they substituted the actual tetragrammaton (which is only the consonants of the Divine Name “YHWH” since Hebrew is not usually written with vowels included) with the word Adonai (Lord). Even in the Septuagint (the Greek version of the Old Testament) the translators substituted Kurios (Lord) for the Divine Name. Eventually the vowels from Adonai (“Lord”) or Elohim (“God”) found their way into the consonants YHWH, thus forming “YaHWeH.” But this does not mean that was how God’s Name was originally pronounced.
Any number of vowel combinations are possible, and the Jews are as uncertain of the real pronunciation as are Christians. “Jehovah” is actually a much later (probably 16th century) variant in Latin. Here, the “Y” is substituted with a “J” (Hebrew does not even have a “J” sound), and the “W” with a “V,” plus another vowel combination, resulting in “JeHoVaH.” This vowel combination is composed of the abbreviated forms of the imperfect, the participle, and the perfect of the Hebrew being verb (English “is”) - thus the meaning of Jehovah could be said to be "he who will be, is, and has been."
So, what is God's name and what does it mean? The most likely choice for how the tetragrammaton was pronounced is "Yahweh" or something very similar to that. The name "Yahweh" refers to God's self-existence. "Yahweh" is linked with how God described Himself in Exodus 3:14, "God said to Moses, 'I AM WHO I AM.' This is what you are to say to the Israelites: 'I AM has sent me to you.'" God's name is a reflection of His being. God is the only self-existent / self-sufficient Being in the universe. Only God has life in and of Himself. That is the essential meaning of the tetragrammaton / YHWH / Yahweh.