In plain words
Jñāna is covered by this constant enemy of the wise, son of Kunti—desire, which is insatiable like fire.
Word for word
Knowledge is covered by this constant enemy of the wise, in the form of desire, O son of Kuntī, hard to fill and never sated.
The commentators
Bhagavadgītābhāṣyam
किं पुनस्तत् इदंशब्दवाच्यं यत् कामेनावृतमित्युच्यते —
आवृतम् एतेन ज्ञानं ज्ञानिनः नित्यवैरिणा, ज्ञानी हि जानाति ‘अनेन अहमनर्थे प्रयुक्तः’ इति पूर्वमेव । दुःखी च भवति नित्यमेव । अतः असौ ज्ञानिनो नित्यवैरी, न तु मूर्खस्य । स हि कामं तृष्णाकाले मित्रमिव पश्यन् तत्कार्ये दुःखे प्राप्ते जानाति
‘तृष्णया अहं दुःखित्वमापादितः’ इति, न पूर्वमेव । अतः ज्ञानिन एव नित्यवैरी । किंरूपेण ? कामरूपेण कामः इच्छैव रूपमस्य इति कामरूपः तेन दुष्पूरेण दुःखेन पूरणमस्य इति दुष्पूरः तेन अनलेन न अस्य अलं पर्याप्तिः विद्यते इत्यनलः तेन च ॥ ३९ ॥
In plain words
And what is this 'this'? What is the thing the verse calls covered over by desire? —
Knowledge gets covered over by this, the eternal enemy every knower carries. Because the one who knows sees it coming — 'this is about to drag me into trouble' — and lives with a steady undertone of sorrow just from seeing that clearly. That's exactly why desire is the eternal enemy of the wise, but not of the fool. The fool treats desire like a friend the whole time craving is alive, and only recognizes it as the enemy once the pain it was always going to cause finally arrives —
— saying only then, 'craving has brought me to grief,' never seeing it coming. That is exactly why this enemy belongs to the wise alone. And what shape does it wear? The shape of desire — desire is nothing but longing itself, wearing that shape. It is hard to fill, because satisfying it never comes easy. And it is insatiable, fire-like — because for desire, there is simply no such thing as enough.
Close to the Sanskrit
But what, further, is that thing denotable by the word 'this', which is said to be covered by desire? —
Knowledge is covered by this, the eternal enemy of the knower. For the knower knows in advance: 'I am being drawn by this into misfortune.' And he becomes constantly sorrowful. Therefore this [desire] is the eternal enemy of the knower, but not of the fool. For he [the fool], regarding desire as a friend at the time of craving, knows [it as an enemy] only when its result — sorrow — has arrived,
'I have been reduced by craving to a state of suffering' — not beforehand at all. Therefore it is of the knower alone that it is the eternal enemy. In what form? In the form of desire — desire, longing itself is its form, hence 'having the form of desire.' By that [enemy] which is hard to fill — its filling happens only with difficulty, hence 'hard to fill.' By that insatiable one too — for it, no sufficiency exists, hence 'insatiable' — by that too.