In plain words
One whose happiness is within, whose delight is within, and whose light is within alone—that yogin becomes Brahman and attains Brahma-nirvāṇa.
Word for word
Who is happy within, delights within, and likewise has his light within alone, that yogin, having become Brahman, attains Brahma-nirvāṇa.
The commentators
Bhagavadgītābhāṣyam
कथम्भूतश्च ब्रह्मणि स्थितः ब्रह्म प्राप्नोति इति आह भगवान् —
यः अन्तःसुखः अन्तः आत्मनि सुखं यस्य सः अन्तःसुखः, तथा अन्तरेव आत्मनि आरामः आरमणं क्रीडा यस्य सः अन्तरारामः, तथा एव अन्तः एव आत्मन्येव ज्योतिः प्रकाशो यस्य सः अन्तर्ज्योतिरेव, यः ईदृशः सः योगी ब्रह्मनिर्वाणं ब्रह्मणि निर्वृतिं मोक्षम् इह जीवन्नेव ब्रह्मभूतः सन् अधिगच्छति प्राप्नोति ॥ २४ ॥
In plain words
And what is such a person like — the one who, settled in Brahman, reaches Brahman? To this the Lord replies:
"Inwardly happy" — his happiness is inside, in the Self; that is what the word means. "Delighting within" — his enjoyment, his play, his sport is within, in the Self and nowhere else. "Lit from within" — his light, the illumination he sees by, is inside, in the Self alone. The yogī who is like this reaches brahmanirvāṇa: the coming to rest in Brahman, liberation. And he reaches it here, while he is still alive, already being Brahman.
Close to the Sanskrit
And being of what sort does one established in Brahman attain Brahman — thus the Lord says:
He who is antaḥsukha: within, in the Self, is the happiness of whom — he is antaḥsukha. Likewise, he within whom alone, in the Self, is ārāma — enjoying, play — he is antarārāma. So too, he within whom alone, in the Self alone, is jyotiḥ, illumination — he is indeed antarjyotiḥ. He who is such, that yogī attains, reaches, brahmanirvāṇa — cessation in Brahman, liberation — here, even while living, being one who has become Brahman.