The Acropolis and the Parthenon were important to Athens as centers of religious practice. The Acropolis is a hill in Athens. Atop the hill sits the Parthenon, which is a temple that was dedicated to Athena, who was the patron goddess of Athens. Within that temple (an open-sided structure) sacrifices were made to the gods. This was the most important function of this part of the city.
While the explicit purpose of the Parthenon was religious, it also was symbolically important. The Athenians' ability to design and build such a temple was an important symbol of their wealth and power as a polis.