In Sophocles's Antigone, Creon decrees that Polynices's body is to be left unburied on the battlefield, where it will be exposed to the elements and animals. He also states that anyone who attempts to bury Polynices will be put to death. To better understand Creon's choices, we must first examine the events that precede the play.
Oedipus was once the ruler of Thebes. Creon took over as king until Oedipus's sons, Polynices and Eteocles, were old enough to rule. Although Polynices was the eldest and next in line for the throne, Eteocles claimed the throne for himself and exiled his older brother.
Polynices responded by raising an army and attacking his native city, Thebes. The two brothers killed each other in the midst of the battle.
Since Eteocles died defending Thebes, Creon allows him a proper burial with honors.
Polynices is not given the same treatment. Creon considers Polynices to be a traitor because he attacked his own city with outside forces. Because of this perceived betrayal, Creon declares that Polynices will not be granted a proper burial (or any burial at all, for that matter).
This decision sets the action of the play into motion, as the titular character is determined for her brother to have a proper burial despite Creon's edict.