In plain words
That which is like poison at first but like nectar in the end, arising from the serenity of Ātma-buddhi, is called sāttvika happiness.
Word for word
That which is like poison at first, like nectar in the end, born of the clarity of one's own buddhi, that happiness is declared sāttvika.
यत् तदग्रे विषमिव परिणामेऽमृतोपमम्तत् सुखं सात्त्विकं प्रोक्तम् आत्मबुद्धिप्रसादजम्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
यत् तत् सुखम् अग्रे पूर्वं प्रथमसंनिपाते ज्ञानवैराग्यध्यानसमाध्यारम्भे अत्यन्तायासपूर्वकत्वात् विषमिव दुःखात्मकं भवति, परिणामे ज्ञानवैराग्यादिपरिपाकजं सुखम् अमृतोपमम् , तत् सुखं सात्त्विकं प्रोक्तं विद्वद्भिः, आत्मनः बुद्धिः आत्मबुद्धिः, आत्मबुद्धेः प्रसादः नैर्मल्यं सलिलस्य इव स्वच्छता, ततः जातं आत्मबुद्धिप्रसादजम् । आत्मविषया वा आत्मावलम्बना वा बुद्धिः आत्मबुद्धिः, तत्प्रसादप्रकर्षाद्वा जातमित्येतत् । तस्मात् सात्त्विकं तत् ॥ ३७ ॥
In plain words
That happiness which feels like poison at the very start — because the first steps of jñāna, vairāgya, meditation, and concentration demand such fierce effort — but which, once it ripens, tastes like nectar: that is the happiness the wise call sattvic. It is born from the clarity of a mind turned toward the Self, a clarity as simple and stainless as clear water. Or, put differently, it comes from a mind fixed on the Self, resting in the Self — or from that very clarity deepening further. Either way, this happiness is sattvic.
Close to the Sanskrit
That happiness which — at first, previously, at the first occurrence, at the beginning of jñāna, vairāgya, meditation, and concentration — because it is preceded by extreme exertion, is like poison, has the nature of suffering; while in the outcome, happiness born from the ripening of jñāna, vairāgya, and the rest, is comparable to nectar — that happiness is declared sattvic by the wise: of the Self, intellect, that is self-intellect; of the self-intellect, the clarity, that is the stainlessness, like the clearness of water; born from that, that is, born from the clarity of the self-intellect. Or, self-intellect may mean the intellect whose object is the Self, or the intellect resting upon the Self; or, born from the intensification of its clarity — this is the meaning. Therefore that happiness is sattvic.