In plain words
When, like a tortoise drawing in its limbs, one completely withdraws the senses from their objects, one's wisdom is firmly established.
Word for word
And when this [one] withdraws the senses from the objects of the senses on every side, as a tortoise its limbs, his wisdom is firmly set.
The commentators
Bhagavadgītābhāṣyam
किञ्च—
यदा संहरते सम्यगुपसंहरते च अयं ज्ञाननिष्ठायां प्रवृत्तो यतिः कूर्मः अङ्गानि इव यथा कूर्मः भयात् स्वान्यङ्गानि उपसंहरति सर्वशः सर्वतः, एवं ज्ञाननिष्ठः इन्द्रियाणि इन्द्रियार्थेभ्यः सर्वविषयेभ्यः उपसंहरते । तस्य प्रज्ञा प्रतिष्ठिता इत्युक्तार्थं वाक्यम् ॥ ५८ ॥
In plain words
And there is more—
"When he withdraws" — and withdrawing here means drawing everything in, completely: this is the ascetic who has settled into knowledge, doing with his senses what a tortoise does with its limbs. Frightened, the tortoise pulls in head and legs, all of them, from every side at once. In just that way, the one grounded in knowledge pulls the senses back from the things they reach for — from every object, without exception. "His wisdom stands firm": that line has already been explained.
Close to the Sanskrit
And what more—
When he withdraws — and 'withdraws' means completely draws in — this ascetic engaged in steadfastness in knowledge, as a tortoise its limbs: just as a tortoise, out of fear, draws in its own limbs wholly, from every side, so the one grounded in knowledge draws in the senses from the objects of the senses, from all objects whatsoever. 'His wisdom is firmly established' — this is a sentence whose meaning has already been stated.