चञ्चलं हि मनः कृष्ण प्रमाथि बलवद्दृढम्। तस्याहं निग्रहं मन्ये वायोरिव सुदुष्करम् ॥

chañchalaṁ hi manaḥ kṛiṣhṇa pramāthi balavad dṛiḍham tasyāhaṁ nigrahaṁ manye vāyor iva su-duṣhkaram

For the mind is verily restless, O Krishna; it is impetuous, strong and difficult to bend. I deem it as hard to curb as the wind.

In plain words: Arjuna says the mind is restless, wild, strong and stubborn. He thinks controlling it is as hard as trying to hold back the wind.