आ॑पो वा॑ इद॑म॑ग्रे सलिल॑मासीत्।
स॑ प्रजा॑पतिः पुश्करपर्णे॑ वा॑तो भूतो᳚ऽलेलायत्।
स॑ प्रतिष्ठां॑ ना॑विन्दत।
स॑ एत॑दपां॑ कुला॑यमपश्यत्।
त॑तो वै॑ स॑ प्र॑त्यतिष्ठत्।
Translation: "In the beginning this (world) was Water, the flood. प्रजा॑पति moved back and forth on a leaf, having become the Wind. He found no support. He saw the home of the Waters. He constructed the Fire. It (the home) became this (Earth). Then indeed He had support."
Word-by-word: आ॑पस् < अ॑प् "Water", वै॑ "indeed", इद॑म् "this", अ॑ग्रे < अ॑ग्र "beginning", सलिल॑म् < सलिल॑ "flood", आसीत् < अस् "be", स॑स् < स॑ "He", प्रजा॑पतिस् < प्रजा॑पति, पुश्कर < पु॑श्कर "flower", पर्णे॑ < पर्ण॑ "leaf", वा॑तस् < वा॑त "Wind", भूत॑स् < भूत॑ < भू "become", अलेलायत् < लेलाय् "move back and forth", स॑स् < स॑ "He", प्रतिष्ठा॑म् < प्रतिष्ठा॑ "support", न॑ "not", अविन्दत < विद् "find", स॑स् < स॑ "He", एत॑द् "this", अपा॑म् < अ॑प् "Water", कुला॑यम् < कुला॑य "home", अपश्यत् < स्पश् "see", त॑स्मिन् < त॑द् "it", अग्नि॑म् < अग्नि॑ "Fire", अचिनुत < चि "construct", त॑द् "it", इय॑म् "this", अभवत् < भू "become", त॑तस् "then", वै॑ "indeed", स॑स् < स॑ "He", प्र॑ति अतिष्ठत् < प्र॑ति स्था "have support".
छ॑न्दस्/Meter: None (the तैत्तिरीयसं॑हिता is mostly prose)
Context: This is one of the many places where the Vedic narrative of the universe's creation is described. प्रजा॑पति is the all-encompassing God of Creation, the closest there is in the Vedic religion to the Abrahamic conception of יהוה/YHWH (though there are still many differences), and He alone created the worlds in the beginning.
This description of Creation is rather elegant in my opinion, with some interesting possible metaphysical interpretations. All four of the classical elements (Water, Wind, Fire, Earth) are mentioned here. Water is the primordial element that existed when there was nothing else (just as in ऋग्वेद॑ 10.129.1). And then there was the motive force, the breath (Wind) of life or the Prime Mover, Who sprang into existence. And the grandson of the Waters (अपां॑ न॑प्तृ) was the Fire, Who is also the light in the Heavens. Finally came the Earth, She Who supports everything else, even प्रजा॑पति Himself.
This narrative is told as part of a section about a ritual called the अग्निच॑यन, the "piling of the fire altar". It comes from the same roots used here: अग्नि॑ "fire" and चि "construct". A similar narrative is found in the famous hymn ऋग्वेद॑ 10.121, where fire (अग्नि॑) is produced from the waters (आ॑पस्) by प्रजा॑पति, who holds up Heaven and Earth (द्यौ॑रुग्रा॑ पृथिवी॑ च दृळ्हा॑) and the Divine light (सु॑वर्) and the Firmament (ना॑क).
Notice the interesting parallels between Vedic creation and the first lines of בְּרֵאשִׁית/Genesis. There, too, we find that in the beginning (בְּרֵאשִׁ֖ית / अग्रे॑) the sole Creator was floating (מְרַחֶ֖פֶת / अलेलायत्) above the waters (הַמָּֽיִם / आ॑पस्), and created a light (א֑וֹר / सु॑वर् or अग्नि॑म्) above them, and lastly created Earth (אֶ֔רֶץ / इय॑म्) from them, having created the Firmament (רָקִ֖יעַ / ना॑कस्).
Interesting Vedic grammar: Notice the extremely abbreviated style of the ब्रा॑ह्मणs, which must be learned through experience. इद॑म् "this" is often a short form for "this world", and इय॑म् (a स्त्रीलिङ्ग/feminine pronoun) a short form for पृथिवी॑ (the Earth). Unfortunately, this style often creates ambiguity—for example, what is त॑द् that became the Earth here? Keith translates it as referring to the अग्नि॑/fire, while I believe it refers to the कुला॑य/home of the waters, since this fits better with the लिङ्ग/gender.