Why is the 'Diya' an important symbol for the Diwali?

Diwali is said to be a festival of lights. It is celebrated on the return of lord Rama and Sita back to their land i.e. Ayodhya after 14 long years' struggle in the forest. So the entire Ayodhya was lightened up to welcome their new king and queen. The Diyas here symbolize victory, glory, the victory of truth over falsehood and evil!