In the epilogue of Wonder Woman, Etta Candy reunites with the boys and convinces them to go on another secret mission, to retrieve an artifact (presumably without Diana?). What is that artifact? There was sketch of what looked like a cube in the folder’s contents she was going through.
That item is (probably) a Mother Box.
The Mother Boxes were left on Earth and the Humans, the Amazons and the Atlanteans each took one of the Mother Boxes to guard. While the Amazons and the Atlanteans kept them safe, the Humans buried their Mother Box so that no-one could ever find it again.
One was used (in the comics) in the creation of Cyborg, hinting at a tie-up between Wonder Woman and Justice League.