Diploblasty is a condition of the ovum in which there are two primary germ layers: the ectoderm and endoderm.
Diploblastic organisms are organisms which evolve from such an ovum, and include cnidaria and ctenophores.
The endoderm allows them to develop true tissue. This includes tissue associated with the gut and associated glands. The ectoderm on the other hand gives rise to the epidermis, the nervous tissue, and if present, nephridia.
Simpler animals, such as sea sponges, have one germ layer and lack true tissue organisation.
All the more complex animals (from flat worms to man) are triploblastic with three germ layers (a mesoderm as well as ectoderm and endoderm). The mesoderm allows them to develop true organs.