In plain words
There one knows an absolute happiness grasped by buddhi and beyond the senses; established there, one does not move from the truth.
Word for word
Where [one] knows that endless happiness which is grasped by buddhi and beyond the senses, and where, established, this one does not move at all from the truth,
The commentators
Bhagavadgītābhāṣyam
किञ्च —
सुखम् आत्यन्तिकं अत्यन्तमेव भवति इत्यात्यन्तिकम् अनन्तमित्यर्थः, यत् तत् बुद्धिग्राह्यं बुद्ध्यैव इन्द्रियनिरपेक्षया गृह्यते इति बुद्धिग्राह्यम् अतीन्द्रियम् इन्द्रियगोचरातीतम् अविषयजनितमित्यर्थः, वेत्ति तत् ईदृशं सुखमनुभवति यत्र यस्मिन् काले, न च एव अयं विद्वान् आत्मस्वरूपे स्थितः तस्मात् नैव चलति तत्त्वतः तत्त्वस्वरूपात् न प्रच्यवते इत्यर्थः ॥ २१ ॥
In plain words
And there is more —
‘Absolute’ happiness means happiness through and through—‘absolute’ really means endless. What's graspable only by the intellect, independent of the senses altogether—that's what ‘graspable by the intellect’ means. ‘Beyond the senses’ means it transcends anything the senses can reach; it isn't born of any object out there. When this knower, standing firmly in his own true nature, tastes such happiness, whenever that happens—he never, ever wavers from it, really. He simply doesn't fall away from his own true nature.
Close to the Sanskrit
And what more —
‘Absolute’ happiness means it is utterly happiness, indeed—‘absolute’ means endless—that is the meaning; that which is graspable by the intellect—i.e. is grasped by the intellect alone, independent of the senses—thus it is ‘graspable by intellect’; ‘beyond the senses’—i.e. transcending the range of the senses, not born of any sense-object—that is the meaning; when this knower, established in his own true nature, knows, i.e. experiences, such happiness, at whatever time—and, at all, he does not, from that, waver, in reality; i.e. he does not fall away from his true essential nature—that is the meaning.