तस्माद्यस्य महाबाहो निगृहीतानि सर्वशः। इन्द्रियाणीन्द्रियार्थेभ्यस्तस्य प्रज्ञा प्रतिष्ठिता ॥

tasmād yasya mahā-bāho nigṛihītāni sarvaśhaḥ indriyāṇīndriyārthebhyas tasya prajñā pratiṣhṭhitā

Therefore, O mighty-armed, whose senses are all completely restrained from the objects of sense, of him the understanding is well poised.

In plain words: So, Arjuna, the person who has fully held back their senses from chasing pleasures has a steady and clear mind.