In plain words
As fire is covered by smoke, a mirror by dust, and an embryo by its membrane, so this knowledge is covered by desire.
Word for word
As fire is covered by smoke, and a mirror by dirt, as the embryo is enclosed by the caul, so this is covered by that.
The commentators
Bhagavadgītābhāṣyam
कथं वैरी इति दृष्टान्तैः प्रत्याययति —
धूमेन सहजेन आव्रियते वह्निः प्रकाशात्मकः अप्रकाशात्मकेन, यथा वा आदर्शो मलेन च, यथा उल्बेन च जरायुणा गर्भवेष्टनेन आवृतः आच्छादितः गर्भः तथा तेन इदम् आवृतम् ॥ ३८ ॥
In plain words
But how is it an enemy? He makes it convincing with examples —
Fire is light by its very nature, and the smoke born out of that same fire — dark by its very nature — covers it over. Or take a mirror, covered by grime. Or the child in the womb, covered, screened out of sight by the amnion — the placenta, the membrane that wraps it round. In just that way, by that this is covered.
Close to the Sanskrit
How is it an enemy — thus, by means of examples, he brings (the hearer) to conviction —
Fire, whose nature is light, is covered by smoke born along with it, whose nature is non-light; or as a mirror is covered by dirt; and as the embryo is covered — concealed — by the amnion, that is, by the placenta, the wrapping of the womb; so by that is this covered.