Bhagavad Gita Verse 19, Chapter 6

yathaa deepo nivaatasthato nengate sopamaa smritaa |
yogino yatachittasya yunjato yogamaatmanaha || 19 ||
 
Just like a lamp in a windless place does not flicker, this state is comparable to the yogi, having controlled the mind, who engages his self in yoga.
 
yathaa : just like
deepaha : lamp
nivaatasthataha : in a windless place
na : does not
ingate : flicker
saa : this (state)
upamaa : comparison
smritaa : has been said
yoginaha : of the yogi
yatachittasya : who has controlled the mind
yunjato : engages
yogam : yoga
aatmanaha : with his self
 
The state of the meditator’s mind is beautifully explained with a classic illustration or “upamaa” by Shri Krishna in this shloka. He compares the mind of a meditator to the flame of a lamp that is burning in a windless room. Just like the flame is unwavering due to absence of wind, so too is the mind of a meditator steady due to absence of desire.
 
If we compare the focused mind to a steady flame, then we need to understand how to prevent to mind from succumbing to the winds of desire. If desires exist, we may be able to temporarily stabilize the mind, but it will eventually get destabilized due to a thought generated by a desire. Presence of desire indicates absence of detachment or vairagya. The only cure is to increase our level of detachment towards the world.
 
The intense level of detachment that is needed for meditation is not an alien concept. When people get extremely absorbed in watching TV, they temporarily develop detachment towards everything else. If we call them for dinner, or if the doorbell rings and so on, these external occurences do not even register in their minds. It again depends on what we give importance to.
 
Now let us examine another aspect of the lamp illustration. According to Vedaanta, seeing an inert object is a 2 step process. First, our mind creates a mental picture of the object. Next, the aatman or self shines a light on that picture, just like a torch does. Only when this light is shined do we come to know that “this is a mango” or “this is a car” and so on.
 
But when we meditate on the self, the second step does not happen because we are not focusing our attention on an inert object. We are focusing on the self, which illuminates itself. It is like shining a torch on the sun - the torch is useless. The self is like the candle since it does not require anything else to illumine it, to shine a light on it. This is another point made in the illustration of the flame.
 
So therefore, when our focus and concentration reaches its peak, and when the target of meditation becomes our own aatmaa or self, it is the culmination of meditation. It is the state of samaadhi. This state is further elaborated upon in the forthcoming shlokas.