“A prosphoron is made up of two separate round pieces of leavened dough which are placed one on top of another and baked together to form a single loaf. This double-loaf represents the two natures of Christ: human and divine. Before baking, each prosphoron is stamped with a seal usually bearing the image of a cross with the Greek letters IC XC NIKA (“Jesus Christ conquers”) around the arms of the cross. This impression is baked into the bread and serves as a guide for the priest who will be cutting it.”
(Monastiriaka.gr site explaining the meaning of prosphora)