He is light of lights.

jyotisamsam api taj jyotis,tamasah param ucyate,
jnanam jneyam jnanagamyam hrdi sarvasya dhisthitam.--17
He is light of lights,said to be beyond darkness,Knowledge,the object of knowledge and the goal of knowledge-he is seated in hearts of all.
Every source of light like sun,stars,takes light from from the supreme because these are creations of god.He is crater of all beings.The light of sun,moon,etc,takes place from that.He is said to be beyond darkness.
jnanam[knowledge]--that which is described in Gita before in 7th solaka as absence of pride,non-violence,patience that which has to be known is called'jneyam and when we find it in our hearts in self is called 'jnanagamyam'.
First of all we hear about that supreme and according to upnisdas we start to maintain these qualities within us.as patience non-violence etc.it is first a theory but when our interest expands then we becomes eager for practical.and when our effort becomes enough we find in our heart[BLISS].To achieve this we should do work hard in saddhana.as maharishi described what is the use of knowing other thing without knowing ourselves.