This is a much-hyped New Age term usually meaning some sort of space or time or interdimensional transportation device created by thought. The Merkaba meditation as instructed by the various parties who purport to teach these things consists among other things of visualizing two counter-rotating tetrahedrons around oneself.
From the Cassiopaea transcripts:
Q: (L) In ancient literature, something called a Merkabah is talked about, but the definition of this extremely mysterious thing has been lost down through the centuries. There have been many "explanations" from such sources as the Midrash - Jewish commentaries - but there is even argument there. It seems that, even then, nobody knew what it was. But now, we have all these New Age folks coming along who have decided that they know what it is, and it is variously described as rotating double tetrahedrons...
A: If no one knows what it is, that is as good as any other explanation.
Q: (L) I want to know what the ancients who wrote about it meant? What is the definition of the word as the ancient writers used it?
A: The original definition predates this.
Q: (L) What is the original definition that predated the ancient writings that we have access to?
A: What do you think?
Q: (L) What? Well, it's a curious word because it is composed of two words or even three: mer kaba or mer ka ba. If we think of it as three part word, we have the Egyptian Ka, which is like the astral body, and the Ba which is similar to the Ka. I guess you could think of them as the astral body and the genetic body. Then there is the Ab which is the sort of principle element of the life in man - like the part that is of God or the soul. The Ab was represented as a red stone. It was the part of the man that expressed desire, lust, courage, wisdom, feeling, sense and intelligence. So, all of them together sort of expresses an abstract creative principle Kaaba is Arabic for cube, and it is the square stone building in which the Black Stone is housed in Mecca. It was supposed to have been built by Ishmael and Abraham. So with Mer, Ka, Ab, and Ba, we have a cube made up of the principle parts of the etheric self, and housing a stone. Soul stone? Mother stone?
A: By god, she's got it!
Q: (L) Okay, we've got the soul or mother stone. Or the mother of all stones. Now that we have a definition, what was it?
A: The Matriarch Stone.
Q: (L) Is the Matriarch Stone the one in Mecca? A: Symbolism reigns supreme here.
Q: (T) Is this also the Philosopher's Stone?
A: "Stone" to those you perceive as ancients symbolized communication from "a higher source."
In a later session, the Merkaba is defined as as an interdimensional device created by 4D STO beings for use in transportation and generally manifesting anything that was needed. The device has been, according to the Cassiopaeans, hidden behind a 'frequency fence' for a long time and various parties are covertly looking for it. The Ark of the Covenant is a separate object, of 4D STS manufacture, according to the same source.