Bhagavad Gita Verse 24, Chapter 5

yontahasukhontaraaraamastathaantarjyotireva yaha |
sa yogi brahmanirvaanam brahmabhootodhigachchati || 24 ||
 
One who finds bliss in the self, who revels in his self, whose knowledge is the self; he is a yogi, and attains liberation through identification with the eternal essence.
 
yaha : one who
antahasukhaha : inner bliss
antaraaraamaha : revels in his inner self
tathaa : and
antarjyotihi : inner light (knowledge)
eva : only
yaha : he who
saha : that person
yogi : true seeker
brahma : eternal essence
nirvaanam : liberated
brahmabhootaha : identified with the eternal essence
adhigachchati : attains
 
Shri Krishna now starts describing how one who is established in the eternal essence eventually attains the eternal essence, in other words, is liberated. The word yogi here refers to a realized seeker. The entire personality of such a seeker is completely turned inward toward the eternal essence. This state is described by three words: antahasukhaha, antaraaraamaha and antarjyotihi.
 
The person who finds bliss within is called antahasukhaha. He does not expect anything from the outside world. The idea that something other than himself will make him happy, that idea has completely gone. To recap a point from the earlier shloka, he experience bliss by seeking positivity within him, it is not negating what is outside. It is like the sun reflected in water. The reflection will never match the brightness of the sun itself. Similarly the joy obtained through the senses is "reflected happiness". But a yogi who has shifted attention to his inner self gets the real deal.
 
Typically, our mind needs to constantly come into contact with an object, person or situation. This  causes agitation in our minds. However, objects, people and situations keep changing. The economy changes, our friends change, our health changes and so on. But the realized seeker is antararaamaha. He revels in himself. Another way to look at this word is "antara-aarama", which means that he rests in himself. He has eliminated all agitations and is completely at ease with himself. There is nothing that compels him to do anything, he just "is".
 
Furthermore, the realized seeker stops looking towards more and more sources of knowledge. He is antarjyotihi. He knows that the eternal essence alone is the ultimate knowledge, the ultimate source of light. In most traditional temples, the only source of light is inside the central room where the deity is located. Symbolically, it depicts the internal light of the self.
 
In this manner, one who is happy in his own self, revels in his own self and finds knowledge in one's own self, such a person is a true enlightened seeker. By such deep identification with the eternal essence, he becomes the eternal essence. In other words, he attains the ultimate liberation or "nirvaana". Liberation is the destruction of all limitations and all notions of finitude. It is the realization that what is in oneself is the same as what is in everything else.