baahyasparsheshvasaktaatmaa vindatyaatmani yatsukham |
sa brahmayogayuktaatmaa sukhamakshayamashrute || 21 ||

One who is not attached to external sense-contacts finds inner bliss. Such a person, established in contemplation of the eternal essence, experiences inexhaustible joy.

baahyasparsheshu : external sense contacts
asaktaatmaa : person without attachment
vindati : finds
aatmani : inner
yat : one who
sukham : bliss
saha : that person
brahmayogayuktaatmaa : established in contemplation of the eternal essence
sukham : joy
akshayam : inexhaustible
ashrute : experiences

So far, Shri Krishna explained the vision of a realized seeker. From this shloka onwards, he starts turning us towards the topic of meditation. Turning oneself inwards, away from the sense objects of the world, is the first step towards meditation. To this end, he explains the reason for turning inward.

Our entire life is geared towards the pursuit of happiness. However, if we introspect, we realize that the happiness we get from sense objects such as tasty food or a visually appealing movie is instantaneous, but fleeting. Sense objects are finite, and so it the happiness that they can provide. Moreover, if we are not careful, we get attached to whatever gives us instant happiness so that we can repeat that experience. This also leads to sorrow, which comes from constant commentary and judgement from intellect when we lose access to sense objects.

But Shri Krishna gives us a better alternative. When we slowly move away from sense objects and turn inward to contemplate on the eternal essence, we can tap into a source of happiness that is much greater than any happiness produced by sense objects. Moreover, this happiness is never-ending, it is inexhaustible, because it is derived out of the eternal essence which is infinite.