इन्द्रियाणां हि चरतां यन्मनोऽनुविधीयते।तदस्य हरति प्रज्ञां वायुर्नावमिवाम्भसि॥
indriyāṇāṃ hi caratāṃ yan mano ’nuvidhīyatetadasya harati prajñāṃ vāyur nāvam ivāmbhasi
Word by word
The verse says
When the mind follows after the senses as they wander, it carries off a man's judgement — the way wind carries off a boat on the water.
In plain words
The image that makes verse 66 concrete.
A boat on open water, and the wind takes it. The boat does not decide to go anywhere. It simply has nothing holding it.
And note the order: the senses wander, and the mind follows them. The mind is not the one in charge.
Where you've felt this · in study
You sit down to do one thing. An hour later you are somewhere entirely different and cannot reconstruct a single step of how you got there. Nothing dramatic happened. You were just never holding the wheel.
Going deeper
The boat image is more precise than it first looks.
A boat is built to be steered. It has a rudder. What the verse describes is not a boat that lacks the equipment but one where nobody is using it, so the wind decides.
That matches the psychology the Gita has been developing. The mind is not portrayed as inherently chaotic. It is portrayed as following — and what it follows is whatever the senses have found. Left to itself it will faithfully go wherever it is pulled.
Harati prajñām — carries off the understanding. The same verb used at 2.60 for the senses carrying off the mind. The chapter keeps returning to this word, and always in the passive: things are taken from you rather than surrendered.
There is also a quiet counterpoint to Chapter 1 here. Arjuna sits in a chariot with a driver holding the reins. The verse describes a boat with nobody at the helm. One of those is a picture of the situation he is in; the other is a picture of the situation he is afraid of.