Moshe asks of Hashem, "behold, I come to the people of Israel, and I will tell them that the G-d of their forefathers sent me to them; and they will ask me, 'What is His name?', how shall I respond? (Shemos 3:13)
This question is highly perplexing. If the words "G-d of your forefathers" is at all meaningful to them, then surely they know His "name"? And is this really the important question they would pose? Wouldn't they focus on trying to get some real evidence that Moshe is genuine and capable of redeeming them, rather than simply finding out His name?
Not only is the request vexing, but the answer is perplexing as well. The name "Eh-keh" is indeed one of the sheimos of Hashem, and yet it appears nowhere else in Tanach besides here. Elsewhere in Tanach the word is used connoting its literal meaning, not as a name of Hashem.
Let us start by understanding the mindset of people before the great events of Yetzias Mitzrayim and Sinai. The idea of a "Great and Mighty Power" was almost universal. Everyone in the world believed in forces that were very powerful, and the question was simply who or what is that force? Every nation had its idols that were considered the "power that be" until another nation bested them, and then the idols of the conquering nations were seen as being the all-powerful forces, and so on. All in all, that is a very logical approach. Just as the nation that wins the battle is seen as being the stronger nation, so too their god is seen as being the stronger god.
The Jewish nation had been enslaved for two centuries, suffering in a most excruciating way. They were slaves, working under torturous conditions, with all sorts of attempts being made to totally eradicate them. Where was the God of their fathers? The only "logical" conclusion was that either He had been bested by the Egyptian gods, or that He was insensitive to Israel's suffering. In either case, He no longer was a viable candidate for being Israel's redeemer, and thus Moshe Rabbeinu's powerful question: how does he explain this to the Jewish People?
Hashem replied that in order to understand the God that is ready to redeem them, they must first relearn what G-d is all about. Their "Elokim" model pictured Hashem as a more-powerful, or even all-powerful, entity, but one for whom, any period of "inactivity" would indicate a shortcoming. If the all-powerful is inactive, he either can't or won't act.
But the real essence of Hashem, is "Y-H-W-H", which means "Was, Is, and Will Be." Hashem transcends time, and to begin to understand Hashem one must be able see the entire picture over a long span of time. Just as a two-dimensional picture (e.g. an X-ray) cannot do justice to a three dimensional object, and just as a single image cannot do justice to an entire movie, so too, man's chronologically segmented grasp of events doesn't properly appreciate and capture Hashem's Providence. If anything one sees a distortion. A farmer who plants a seed, watches it disintegrate, and sees nothing happen all winter, could be mighty disappointed if he lacks the knowledge and foresight to know what will happen in the spring. It is only when we can see all the events in a long sequence that we understand. It is the past, present, and future combined that may yield a more complete understanding of Hashem's hashgacha.
Hashem therefore told Moshe, "Klal Yisroel has knowledge of the past, i.e. the God of their forefathers. They must add the "Eh-keh", i.e. the understanding of the future. Only when they will be able to see the entire continuum in one fell swoop, will they be able to perceive Hashem's providence and benevolence which is to be found even in the present!