प्रणवो धनुः शरो ह्यात्मा ब्रह्म तल्लक्ष्यमुच्यते ।अप्रमत्तेन वेद्धव्यं शरवत्तन्मयो भवेत् ॥ ४ ॥
In plain words
Praṇava is the bow, Ātma the arrow, and Brahman is said to be its target. It must be pierced by one who is vigilant; one should become made of it, like the arrow.
The commentators
Muṇḍakopaniṣadbhāṣyam· bhāṣya
यदुक्त धनुरादि, तदुच्यते—प्रणव ओंकार धनु । यथा इष्वासन लक्ष्ये शरस्य प्रवेशकारणम्, तथा आत्मशरस्याक्षरे लक्ष्ये प्रवेशकारणमोकार । प्रणवेन ह्यभ्यस्यमानेन संस्क्रियमाणस्तदालम्बनॊऽप्रतिबन्धेनाक्षरेऽवतिष्ठते । यथा धनुषा अस्त इषुर्लक्ष्ये । अत प्रणवो धनुरिव धनु । शरो ह्यात्मा उपाधिलक्षण पर एव जले सूर्यादिवदिह प्रविष्टो देहे सर्वबौद्धप्रत्ययसाक्षितया, स शर इव स्वात्मन्येवार्पितोऽक्षरे ब्रह्मणि, अत ब्रह्म तत् लक्ष्यमुच्यते लक्ष्य इव मन समाधित्सुभिरात्मभावेन लक्ष्यमाणत्वात् । तत्रैव सति अप्रमत्तेन बाह्यविषयोपलब्धितृष्णाप्रमादवर्जितेन सर्वतो विरक्तेन जितेन्द्रियेणैकाग्रचित्तेन वेद्धव्य ब्रह्म लक्ष्यम् । ततस्तद्वेधनादूर्ध्वं शरवत् तन्मय भवेत्, यथा शरस्य लक्ष्यैकात्मत्व फल भवति, तथा देहाद्यात्मप्रत्ययतिरस्करणेनाक्षरैकात्मत्व फलमापादयेदित्यर्थ ॥
In plain words
The passage now unfolds the image of bow and arrow. Praṇava, the syllable Oṃ, is the bow. A bow drives an arrow into its target; practiced steadily, Oṃ carries the arrow of Ātman into the Akṣara. Resting on Praṇava and refined by its repetition, Ātman enters the Akṣara without obstruction, as an arrow enters its mark. The arrow is Ātman: the supreme reality appearing through Upādhi within this body, like the sun reflected in water, yet always the witness of every movement of Buddhi. This arrow is placed in its own true Ātman, the Akṣara-Brahman. Brahman is called the target because those who gather Manas into stillness aim at it as their own Ātma-bhāva, their very being. The one who pierces it must be vigilant, free of thirst for outer experience, wholly dispassionate, master of the senses, and one-pointed in Citta. Once the target is pierced, the arrow becomes one with it. In the same way, when the false conviction that body and the rest are Ātman is set aside, what remains is oneness with the Akṣara.
Close to the Sanskrit
What was stated about the bow and the rest is now explained. Praṇava, Oṃkāra, is the bow. Just as the bow causes an arrow to enter its target, Oṃkāra causes the arrow that is Ātman to enter its target, the Akṣara. Refined through repeated practice of Praṇava and resting upon it, Ātman comes to stand unobstructed in the Akṣara, as an arrow shot from a bow stands in its target. Therefore Praṇava is a bow in the likeness of a bow. Ātman indeed is the arrow: the supreme itself, indicated through Upādhi, has entered this body like the sun reflected in water and is the witness of every cognition of Buddhi. Like an arrow, it is to be placed in its own Ātman, Akṣara-Brahman. Therefore Brahman is called the target: those who wish to concentrate Manas aim at it as their own Ātma-bhāva, their very being. That target, Brahman, must be pierced by one who is not heedless—free from the heedlessness that is thirst for experience of external objects, dispassionate in every direction, master of the indriyas, with Citta one-pointed. After piercing it, one should become of its nature like the arrow. As the arrow’s result is oneness with its target, so, by setting aside the notion that body and the rest are Ātman, one brings about the result of oneness with the Akṣara. This is the meaning.