A Mother Box is Alien and cube-shaped, and generally thought to be sentient, miniaturized, portable supercomputers. They are the vastly more advanced technology of New Gods.
Mother Boxes were created in a different dimension to the rest of the galaxy on the planets New Genesis and Apokolips by the New Gods, greatly technologically advanced beings.
Ages ago, Steppenwolf took three mother boxes to Earth to invade the planet by combining the three mother boxes it would turn the planet into a apokoliptian state, but after the Olympian Gods, Amazons, Atlanteans, Green Lanterns, and ancient human civilazation fought off Steppenwolf and his army, there were three were hidden on the distant planet to be kept from the reach of Darkseid. One was given to the Humans, one to the Amazons, and the final one to the Atlanteans beneath the ocean. They were to be kept and hidden and eventually forgotten about for all of time.