The merkaba, sometimes spelled merkabah, is a (higher dimensional) energy field with a geometric appearance. Often it takes the shape of two interlocked tetrahedrons, which makes it look like a three-dimensional Star of David. It can also appear as a ball of light, which sometimes takes the shape of an egg.
Every living being is surrounded by such an energy field, which means everybody has a merkaba. It can be used for astral and interdimensional travelling. This, however, requires a certain level of consciousness and mastery of spiritual and mental faculties which few people attain. For this reason, some alien civilisations created an artificial or external merkaba: a synthetic energy field to travel through time and space using higher dimensions. It is believed they shared this technology with some secret societies here on earth.
Nowadays, most teachings with regard to the merkaba are linked to Sacred Geometry. It should be pointed out, however, that many shamans have this knowledge, too.