बाह्यस्पर्शेष्वसक्तात्मा विन्दत्यात्मनि यत्सुखम्। स ब्रह्मयोगयुक्तात्मा सुखमक्षयमश्नुते ॥

bāhya-sparśheṣhvasaktātmā vindatyātmani yat sukham sa brahma-yoga-yuktātmā sukham akṣhayam aśhnute

He, whose self is unattached to external contacts and findeth joy in the Self, having the self harmonised with the Eternal by yoga, enjoys imperishable bliss.

In plain words: Krishna says that a person who does not cling to outside pleasures finds joy inside, in their true self. Through yoga, meaning practice that joins the mind to God, they feel a happiness that never ends.