The Holy Quran tells us that Allah sent down two things; one a light and the other the Holy Quran.
What then is the light that was sent by Allah?
That light was the Holy Prophet Muhammad (pbuh). He is the light because the Holy Prophet brought people from darkness into light and because he shed light on how to act on the commandments of the Holy Book. The Quran commands us to pray and the Holy Prophet showed us how to pray, for example.