On four places, we can see the struggle between a bull and a lion. This is a very ancient Iranian motif, known from the fourth millennium BCE, and therefore belonging to the age before the invasion of the Indo-Iranians (the "Aryans").
It symbolizes (probably) eternity. The same message is more or less implied in the representation of the sun. It is an excellent theme for this place. The people who visited the Apadana offered tribute to the great king and received presents in return. This reciprocity strengthened the ties between the ruler and the subjects, and contributed to the continuation of the empire.