The main point of the kaddish is the congregational response:

"May His great name be blessed forever and ever and ever"

You could say that the entire Kaddish is just a frame for this response—in other words, we say the Kaddish so that the congregation can answer with these words.

So what's the big deal with these words? Here's a tip-of-the-iceberg explanation:

His great name refers to the four-letter name that cannot be erased. We never pronounce that name the way it is written. The only time it was pronounced was in the priestly blessing in the Temple in Jerusalem and once a year by the High Priest when he entered the Holy of Holies on Yom Kippur.

This "great name" has many meanings. It has the meaning of was, is and will be all at once. It also has the meaning of that which causes being. It is telling us that G_d encompasses all of existence and yet remains entirely beyond all space and time.

When we say that this great name should "be blessed", we mean that it should become real in our world. That's the meaning of the word blessing in Hebrew (bracha)—it means to bend something downward. In this case, we want to draw that higher reality downward into our reality. We want to feel how everything in the universe is permeated with G_dliness—with the light of an infinite Creator that transcends all things.

Why do we repeat ourselves, saying forever and ever and ever? Isn't one "forever" enough?

The clue could be in the Aramaic word we use (l'alam). There's another meaning to this word, besides forever—it can also mean to the world. According to the Kabbalah, there are many worlds on higher and higher spiritual levels than our own. These worlds aren't somewhere in outer space—they are here in the same place as we are. We can't perceive them, because we are physical beings with physical eyes and ears. But these worlds affect us, nonetheless—and our actions, words and thoughts affect them.

So when we say forever and ever and ever we also mean into all the worlds. The created worlds are divided into three categories Creation, Formation and Action—so that's the reason for repeating three times.1