Yahuweh

His name was first revealed to Moses.
Exodus 6:3 And I appeared unto Abraham, unto Isaac, and Jacob, by the name of El Shaddai (God Almighty), but by my name Yahuweh was I not known to them.

This ineffable name derived from the substantive verb hwhy, hayah, to be; and combining, as it does, in its formation the present, past, and future significations of the verb, it is considered as designating God in His immutable and eternal existence. The four letters, of God's name, may be so arranged by permutations as to form twelve words, every one of which is a modification of the verb to be, and hence it is called the name of His substance or existence.
His name was called the Shem hamphorash, the explanatory or declaratory name, because it alone, of all the Divine names, distinctly explains or declares what is the true essence of the deity.
Among the Essenes, this sacred name, which was never uttered aloud, but always in a whisper, was one of the mysteries of their initiation, which candidates were bound by a solemn oath never to divulge.
The word Yahuweh hwhyis derived from the substantive verb hyh, hayan,  to be, and in its four letters combines those of the past, present and future of the verb. The letter y in the beginning is a characteristic of the future; the w in the middle, of the participle or present time; and the  h at the end, of the past. Thus, out of  hwhy we get hyh, he was; hwh, he is; and hyhy, he will be. Hence, among other titles it received that of nomen essentice, because it shows the essential nature of God's eternal existence. The other names of God define His power, wisdom, goodness, and other qualities; but this alone defines His existence.
Why than doesn't anyone speak or say (the name)?
In the third chapter of Exodus, when Moses asks of God what is His name, He replies "I Am that I Am"; and He said, Thus shalt thou say unto the children of Israel, I Am is EEhyeha, Ehyeh. But as Mendelsohn has correctly observed, there is no essential difference between ehyeha, in the sixth chapter and hwhy in the third, the former being the first person singular, and the latter the third person of the same verb (the future used in the present sense of the verb to be); and hence what was said of the name Ehyeh was applied by the Rabbis to the name Yahuweh. But of Ehyeh God had said, "this is My name forever." Now the word forever is represented in the original by forever, l'olam; but the Rabbis, says Capellus, by the change of a single letter, made l'olam, forever, read as if it had been written l'alam, which means "to be concealed," and hence the passage was translated "this is my name to be concealed," instead of "this is my name forever." And thus Josephus, in writing upon this subject, uses the following expression: "Whereupon God declared to Moses His holy name, which had never been discovered to men before; concerning which it is not lawful for me to say any more." In obedience to this law, whenever the word Yahuweh occurs to a Jew in reading, he abstains from pronouncing it, and substitutes the word Adna , Adonai.
This is opinion based on history not of God!