All auspicious functions commence with the lighting of the Diya lamp, this is often maintained right through the occasion, festival, ritual or pooja. Light symbolizes knowledge and darkness represents ignorance. Ishwar/Bhagwan(God) is the principle of knowledge who is the source, the enlivener and the illuminator of all knowledge. Knowledge removes ignorance just as light removes darkness. Knowledge is a lasting inner wealth by which all outer achievement can be accomplished. Hence we light the lamp to bow down to the knowledge as the greatest of all forms of wealth. The oil or ghee that is used to light up the diya(lamp) symbolize our vaasana or negative tendencies and pride/ego. When lit by spiritual knowledge, the vaasana get exhausted and the ego finally perishes. Similarly we should acquire such knowledge as to take us towards higher ideals.
Deepajypthi parambrahma
Deepa sarva tamopahaha
Deepana saadhyate saram
Sandhyaa deepo namostute
I prostrate to the dark/dusk lamp; whose light is the knowledge principle (the supreme being), which removes the darkness of ignorance and by while all can be achieved in life.