yunjannevam sadaatmaanam yogi niyatamaanasaha |
shaantim nirvaanaparamaam matsamsthaamadhigacchati || 15 ||

In this manner, the yogi who has subdued his mind, who always engages his self in me, attains ultimate liberation-bearing peace, established in me.
yunjan : engaged
evam : in this manner
sadaa : always
aatmaanam : self
yogi : yogi
niyatamaanasaha : one who has controlled his mind
shaantim : peace that leads to
nirvaanaparamaam : ultimate liberation-bearing
matsamsthaam : established in me
adhigacchati : attains
Previously, Shri Krishna explained the prerequisites and the method of meditation. Now, he speaks about the result or the fruit of meditation. He says that meditation, if followed as the technique prescribed here, brings us that peace that yields self-realization.
The journey towards this end goal obviously will take a long time. But there are intermediate results along the way. One who begins to drop attachment and fascination for material objects attains a state of ever-increasing peace. However, this peace does not lead to liberation. It is not “nirvaana paramam”.
Only peace gained by meditating upon the self leads to liberation. Initially, the sense of peace is only present while meditating, but slowly remains with the seeker for longer periods of time. Till the final stage is reached, the person may falter in his journey. Once the final stage is reached, he will never turn back.
Now, what is the source or this peace? Shri Krishna says that it is he who is the source of this peace. One of the fundamental lessons of the Gita is that only the eternal essence can give everlasting bliss and peace. Everything else gives temporary peace. Therefore, Shri Krishna urges the seeker to comprehend this fact and stop going after objects in the material world for happiness and peace.