In plain words
Wherever restless and unsteady manas wanders, let one restrain it from there and bring it under control in Ātma alone.
Word for word
From whatever [cause] the restless, unsteady mind wanders out, from that, having restrained this [mind], one should bring [it] under control in the Self alone.
यतो यतो निश्चरति मनश्चञ्चलमस्थिरम्ततस्ततो नियम्यैतदात्मन्येव वशं नयेत्
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
तत्र एवमात्मसंस्थं मनः कर्तुं प्रवृत्तो योगी —
यतो यतः यस्माद्यस्मात् निमित्तात् शब्दादेः निश्चरति निर्गच्छति स्वभावदोषात् मनः चञ्चलम् अत्यर्थं चलम् , अत एव अस्थिरम् , ततस्ततः तस्मात्तस्मात् शब्दादेः निमित्तात् नियम्य तत्तन्निमित्तं याथात्म्यनिरूपणेन आभासीकृत्य वैराग्यभावनया च एतत् मनः आत्मन्येव वशं नयेत् आत्मवश्यतामापादयेत् । एवं योगाभ्यासबलात् योगिनः आत्मन्येव प्रशाम्यति मनः ॥ २६ ॥
In plain words
So here is the yogī who has begun, just in this way, the work of settling the mind in the Self —
Whatever draws the mind out—a sound, or anything like it—the mind wanders after it, restless by its very nature, endlessly moving, and for that very reason unsteady. From whatever cause draws it out, pull it back: see straight through that cause to what it really is, until it's reduced to a mere flicker, and by cultivating dispassion, bring this mind under control, in the Self alone—make it answer to the Self. This way, through the strength gained from practicing yoga, the yogī's mind grows calm, resting nowhere but in the Self.
Close to the Sanskrit
There, the yogī who has thus set himself to making the mind established in the Self —
From whatever cause—such as sound and so on—the mind wanders out, i.e. goes out, owing to the fault of its own nature, restless, exceedingly moving—precisely for this reason, indeed, unsteady—from that very cause, such as sound and so on, having restrained it, having made that particular cause appear as a mere semblance by ascertaining its true nature, and by the cultivation of dispassion, one should bring this mind under control in the Self alone, i.e. should bring about its state of being subject to the Self. Thus, through the power of the practice of yoga, for the yogī, the mind becomes calm in the Self alone.