Verse 1.4.3

तानु तत्र मृत्युर्यथा मत्स्यमुदके परिपश्येदेवं पर्यपश्यदृचि साम्नि यजुषि । ते नु विदित्वोर्ध्वा ऋचः साम्नो यजुषः स्वरमेव प्राविशन् ॥ १.४.३ ॥

tānu tatra mṛtyuryathā matsyamudake paripaśyedevaṃ paryapaśyadṛci sāmni yajuṣi | te nu viditvordhvā ṛcaḥ sāmno yajuṣaḥ svarameva prāviśan || 1.4.3 ||

3. Just as a person can see a fish swimming in shallow water [i.e., the fish is exposed to the risk of being caught], in the same way, Death could see the gods and goddesses when they depended on Vedic rituals [i.e., they were in easy reach of Death]. Realizing this, the gods and goddesses switched over to the recitation of Om.

Word-for-word explanation:

Yathā, just as; udake, in shallow water; matsyam, a fish [swimming]; paripaśyet, a person can see; evam, in the same way; tatra, in those [Vedic rites and rituals]; ṛci sāmni yajuṣi, in the Ṛg, Sāma, and Yajur Vedas; tān, those gods and goddesses; mṛtyuḥ, Death; paryapaśyat, saw [i.e., they could not escape death through Vedic rituals]; te, they [the gods and goddesses]; nu viditvā, having realized [that they were still susceptible to death]; ṛcaḥ sāmnaḥ yajuṣaḥ ūrdhvāḥ, turned away from Ṛk, Sāma, and Yajur rituals; svaram eva prāviśan, took to [reciting] Om.

Commentary:

Fish in shallow water are never safe, for people can easily catch them. Similarly, those who depend on karma (i.e., Vedic rituals) are always liable to being caught by death. When people realize this, they stop performing the rites and rituals and concentrate on reciting Om. They know that Om is a symbol of immortality and fearlessness.