In plain words
When the mind follows the roaming senses, it carries away one's wisdom just as the wind carries a boat upon the waters.
Word for word
For the mind which follows the roaming senses, that carries off his wisdom, as the wind a boat on the water.
The commentators
Bhagavadgītābhāṣyam
अयुक्तस्य कस्माद्बुद्धिर्नास्ति इत्युच्यते —
इन्द्रियाणां हि यस्मात् चरतां स्वस्वविषयेषु प्रवर्तमानानां यत् मनः अनुविधीयते अनुप्रवर्तते तत् इन्द्रियविषयविकल्पनेन प्रवृत्तं मनः अस्य यतेः हरति प्रज्ञाम् आत्मानात्मविवेकजां नाशयति । कथम् ? वायुः नावमिव अम्भसि उदके जिगमिषतां मार्गादुद्धृत्य उन्मार्गे यथा वायुः नावं प्रवर्तयति, एवमात्मविषयां प्रज्ञां हृत्वा मनो विषयविषयां करोति ॥ ६७ ॥
In plain words
Why is it that the one who is not yoked has no such discernment? Here is the answer —
Here is why: the senses roam, each one chasing its own object, and whichever mind trails along after them gets stirred up by that constant flickering from one sense-object to the next. And that stirred-up mind steals away the striver's wisdom — the insight born from clearly telling Self apart from non-Self — and destroys it. How does this happen? Think of a boat on the water, headed somewhere, and a wind that wrenches it off its course and drives it astray. In just the same way, this restless mind carries off wisdom that was once focused on the Self, and turns it instead toward objects.
Close to the Sanskrit
Why is there no discerning intellect for one who is unyoked? Thus it is said —
For, since, of the senses as they roam, each engaging in its own object, whichever mind follows, follows along [with them] — that mind, set in motion by the alternation among sense-objects, carries off this striver's wisdom, born of the discrimination of Self and non-Self; it destroys it. How? As wind, having pulled a boat on the water away from the course of those wishing to go [somewhere], drives that boat off course — just so, having carried off wisdom whose object is the Self, [the wind-like sense-activity] makes the mind's [wisdom instead] have objects as its object. [End of commentary on verse 67.]