Verse 2.10.5

एकविंशत्यादित्यमाप्नोत्येकविंशो वा इतोऽसावादित्यो द्वाविंशेन परमादित्याज्जयति तन्नाकं तद्विशोकम् ॥ २.१०.५ ॥

ekaviṃśatyādityamāpnotyekaviṃśo vā ito'sāvādityo dvāviṃśena paramādityājjayati tannākaṃ tadviśokam || 2.10.5 ||

5. With the help of those twenty-one syllables, one can attain the status of the sun [which is also Death]. The sun occupies the twenty-first place after the things that come between the earth and the sun [those things being the twelve months, the five seasons, and the three worlds]. One can then go beyond the sun if one knows the twenty-second syllable. That world is full of joy and free from all sorrows.

Word-for-word explanation:

Ekaviṃśatyā, by twenty-one [syllables]; ādityam āpnoti, one attains union with the sun [because the sun marks the end of everything, and it is therefore death]; ekaviṃśaḥ vai, [the sun is] the twenty-first [after the twelve months, the five seasons, and the three worlds]; itaḥ, from this [world]; asau ādityaḥ, the sun over there; dvāviṃśena, by [knowing] the twenty-second [syllable]; ādityāt param jayati, one attains the next higher world from the sun; tat nākam, that [place is] joyful; tat viśokaṃ, that [place is] free from all suffering.

Commentary:

The Upaniṣad says that if you worship Sāma with these twenty-one syllables, you attain the sun, which is the same as death. But how is the number significant? It is significant because the sun occupies the twenty-first position after the things that intervene between the earth and the sun. According to the Vedas those things are the twelve months, the five seasons, and the three worlds. But if you worship the Sāma with twenty-two syllables, you then go beyond the sun. Where? To a place called Nāka, where there is only happiness and no suffering.