Though it sounds like it could be the name of a distant planet in a sci-fi novel, the Tetragrammaton is actually the arrangement in Hebrew of the letters said to represent the very name of God, as first revealed by God himself to Moses in the Old Testament. Rabbi Jacobs says, "The Tetragrammaton is the four-letter name of God formed from the letters yod, hey, vav, and hey, hence YHVH in the usual English rendering." Most commonly the divine name is translated into English letters as YHWH, and sometimes the vowels A (some say for another divine name, "Adonai") and E (for "Elohim") are inserted between the two H's to make the more readily pronounceable "Yahweh."
"The original pronunciation of the Tetragrammaton," continues Jacobs, "has been lost, owing to the strong Jewish disapproval of pronouncing the name. The pronunciation Yahveh or Yahweh is based on that used by some of the Church Fathers but there is no certainty at all in this matter." This is a rather striking fact, that a community of many millions of people would be so averse to uttering a certain name for God that its pronunciation was finally forgotten and left a complete mystery to succeeding generations. All this naturally leads to the question: "Just why were the ancient Hebrews so reluctant to use that name?"
One possible answer is found in a passage from Leviticus:
“Then you shall speak to the children of Israel, saying: ‘Whoever curses his God shall bear his sin. And whoever blasphemes the name of the Lord shall surely be put to death. All the congregation shall certainly stone him, the stranger as well as him who is born in the land. When he blasphemes the name of the Lord, he shall be put to death" (Lev. 24:15-16).
That helps explain things, in that the Jews weren't keen on the idea of breaking the laws of God that carried the death penalty. But certain Israelites were evidently willing to run that risk, at least when it came to things like breaking the Sabbath, committing adultery or cursing their parents. The question remains why the particular injunction against blasphemy was treated with such extreme seriousness. To answer that we may need to trace further back to the beginnings of the Hebrew nation, to God's revelation to Moses at the burning bush. There Moses directly asked God for a name to identify him before Pharaoh and to distinguish God clearly from the gods of Egypt:
And God said to Moses, "I AM WHO I AM." And He said, "Thus you shall say to the children of Israel, "I AM has sent me to you" (Exodus 3:14).
There the Tetragrammaton first appears. According to Jewish scholars the name refers primarily to being, and thus means something like, "I am that I am," or, "I will be that I will be." Jacobs adds that while names like Elohim are used of God in a universal sense, "the Tetragrammaton is used of God in His special relationship with Israel." In other words this is the name God uses of himself in a highly personal way – which implies that if that name were to be misused God would likely "take it personally."
Sometime later, following the Exodus from Egypt, the fledgling Israelite nation found herself in direct contact with God himself at Sinai in the wilderness. There God met with Moses, as the mountain burned with fire and the people trembled in fear below:
Now all the people witnessed the thunderings, the lightning flashes, the sound of the trumpet, and the mountain smoking; and when the people saw it, they trembled and stood afar off. Then they said to Moses, "You speak with us, and we will hear; but let not God speak with us, lest we die" (Ex. 20:18-19).
From all indications, then, the Israelites' abhorrence of the particular sin of blasphemy arose from their experience, and ongoing national memory, of God's holy presence in the wilderness. So frighteningly powerful was this experience that they collectively decided to take no chances, and in a demonstration of reverence left off pronouncing God's name altogether. This makes Israel unique, in being the only nation on earth whose present existence cannot be adequately explained apart from some powerful historical interactions with the Almighty. That being the case, the reality of YHWH, the God of Israel, best explains the origins of Israel. As John Bright argues, “The history of Israel is the history of a people which came into being at a certain point in time as a league of tribes united in covenant with Yahweh… Since this is so, Israel’s history is a subject inseparable from the history of Israel’s religion."
Fast-forwarding some 1500 years from the Exodus brings us to the time of Jesus and the apostles, and another set of unexpected divine-human historical interactions. God again appears to his people, this time in a resurrected body following three years of teaching and miracle ministry culminating in a brutal crucifixion at the hands of Roman authorities (and at the instigation of jealous religious leaders). This time he appears to a handful of men and women, whom he charges with the responsibility of preaching the gospel (good news) of his kingdom throughout the world. Again the people chosen are (mostly) Jews, and again history changes course to reflect the will of God.
The Israelites were afraid to so much as utter the name of YHWH. Now God's people, having seen Jesus risen and having been filled with the Holy Spirit at Pentecost, boldly preach God's salvation in the name of Jesus Christ to anyone who willing to listen. This saving ministry of Christ is all the more astounding when we realize that Jesus was crucified not for committing crimes or acts of sedition against the state, but for committing the ultimate blasphemy, that is, claiming for himself the very identity of the God of smoke and fire who met with Moses on the mountain so many centuries before. "Before Abraham was," Jesus declared to some very powerful and pious religious authorities, "I AM" (John 8:58).
At the cost of their livelihoods, their reputations, and even their lives, this small band of disciples thus managed to spread the gospel throughout the first century world. Just as God's revelation to Moses arguably best explains the origins of national Israel, so the post-mortem appearances of Jesus to his disciples seem to best explain the rise of the early church. As a result people like me to this day can confidently proclaim that the God of Abraham has acted again in history; that Jesus died for our sins and rose from the dead.
 Rabbi Louis Jacobs, "The Tetragrammaton," My Jewish Learning, http://www.myjewishlearning.com/article/the-tetragrammaton/.
 Jacobs, "The Tetragrammaton."
 Crawford Howell Toy & Ludwig Blau, "Tetragrammaton," Jewish Encyclopedia, http://www.jewishencyclopedia.com/articles/14346-tetragrammaton
 Jacobs, "The Tetragrammaton."
 John Bright, A History of Israel (Louisville: Westminster John Knox Press, 2000), p. xvii.