Meaning behind lighting a lamp in house :-

Why do we light a lamp/diya/deepam in our house daily?

In almost every Indian home a lamp is lit daily before the altar of the Lord. In some houses it is lit at drawn, in some, twice a day – at dawn and dusk and in a few it is maintained continuously (akhanda deepa). All auspicious functions and moments like daily worship, rituals and festivals and even many social occasions like inaugurations commence with the lighting of the lamp, which is often maintained right through the occasion.

• Why do we light a lamp?

Light symbolizes knowledge and darkness, ignorance. The Lord is the “Knowledge Principle” (chaitanya) who is the source, the enlivener and the illuminator of all knowledge. Hence light is worshipped as the Lord Himself.

Knowledge removes ignorance just as light removes darkness. Also knowledge is a lasting inner wealth by which all outer achievements can be accomplished. Hence we light the lamp to bow down to knowledge as the greatest of all forms of wealth. Knowledge backs all our actions whether good or bad. We therefore keep a lamp lit during all auspicious occasions as a witness to our thoughts and actions.
Why not light a bulb or tube light? That too would remove darkness. But the traditional oil lamp has a further spiritual significance. The oil or ghee in the lamp symbolizes our vaasanas or negative tendencies and the wicked, the ego. When lit by spiritual knowledge, the vaasanas get slowly exhausted and the ego too finally perishes. The flame of a lamp always burns upwards. Similarly we should acquire such knowledge as to take us towards higher ideals.
A single lamp can light hundreds more just as a man of knowledge can give it to many more. The brilliance of the light does not diminish despite its repeated use to light many more lamps. So too knowledge does not lessen when shared with or imparted to others. On the contrary it increases in clarity and conviction on giving. It benefits both the receiver and the giver.

Whilst lighting the, lamp we thus pray :
Deepajyotihi parabrahma  
Deepa sarva tamopababa
Deepena sadhyate sarvam
Sandhyaa deepo namostute

The meaning is:

I prostrate to the dawn/dusk lamp; whose light is the Knowledge Principle (the Supreme Lord), which removes the darkness of ignorance and by which all can be achieved in life.