सुखमात्यन्तिकं यत्तद्बुद्धिग्राह्यमतीन्द्रियम्।वेत्ति यत्र न चैवायं स्थितश्चलति तत्त्वतः॥
sukham ātyantikaṃ yat tad buddhi-grāhyam atīndriyamvetti yatra na caivāyaṃ sthitaś calati tattvataḥ
Word by word
The verse says
— where he knows that endless happiness which the understanding can hold but the senses cannot reach, and standing there, he does not move from what is real —
In plain words
The second piece of the definition.
Atīndriyam — beyond the senses. Not delivered through the eyes or ears or skin.
And yet buddhi-grāhyam — the understanding can take hold of it. So it is not vague.
Where you've felt this · with money
The best hour of your week costs nothing and involves nobody selling you anything. You have known this for a long time. You still check the shops on Saturday.
Going deeper
Two words are doing opposite jobs here and the pairing is the point.
Atīndriyam places this happiness outside the reach of the senses, which rules out every pleasure that arrives by contact. Buddhi-grāhyam says the understanding can nonetheless get hold of it, which rules out its being merely a feeling or a mood.
So it is neither sensed nor imagined. It is available to the part of you that knows things, and the Gita is content to leave it there rather than describing it further.
That restraint is worth noticing. A text making this up would say more.
And the last clause is the practical one: standing there, he does not move from what is real. The happiness is not the endpoint. Being unable to be talked out of what is actually the case — that is what standing in it produces.