Maaseh Merkavah is the most hidden and esoteric part of Judaism. It means literally the story (or work or discipline) of the Chariot, from the verb rochev, to ride.
Maaseh Merkavah is concerned with interpreting the vision of the angelic chariot that opens the book of Ezekiel, beginning I was among the exiles by the river Kevar, the heavens opened and I saw visions of God.
Although it is the most sustained and vivid description of a mystical vision in the Bible, the Talmud (in tractate Hagigah) says that one should not therefore think that other prophets did not have these, or greater, experiences; on the contrary, the rarity of the event for Ezekiel prompted him to record it in such detail.
The Mishnah (Hagigah 2:1) writes that one may not teach Maaseh Merkavah to more than one student at a time. Even then, one may only teach Maaseh Merkavah to someone who is capable of understanding it on his own; the teacher communicates merely chapter headings, the main outlines of the subject matter, from which the disciple may deduce the rest.
Most believe that the content of the Maaseh Merkavah is mystical, kabbalistic secrets for which the vivid images of Ezekiels vision are symbolic representations.
Maimonides, unusually, identified Maaseh Merkavah with the esoteric teachings of philosophy, while Rabbi Aryeh Kaplan, the modern kabbalistic teacher, traced in Ezekiels vision stages in the experience of mystical meditation.