They don't mention in subtitles, and it's hard for me to understand. (I speak English only.)
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The mantra being chanted is "oṁ namo bhagavate vāsudevāya", which means "Om! Salutations to Bhagavan Vāsudeva." Bhagavan literally means the one who has wealth, but in this context it means "supreme being". And Vāsudeva is a name of Vishnu. In any case, this mantra occurs in the very first verse of the Srimad Bhagavatam, which says this:
oṁ namo bhagavate vāsudevāya
janmādy asya yato ’nvayād itarataś cārtheṣv abhijñaḥ svarāṭ
tene brahma hṛdā ya ādi-kavaye muhyanti yat sūrayaḥ
tejo-vāri-mṛdāṁ yathā vinimayo yatra tri-sargo ’mṛṣā
dhāmnā svena sadā nirasta-kuhakaṁ satyaṁ paraṁ dhīmahi
O my Lord, Śrī Kṛṣṇa, son of Vasudeva, O all-pervading Personality of Godhead, I offer my respectful obeisances unto You. I meditate upon Lord Śrī Kṛṣṇa because He is the Absolute Truth and the primeval cause of all causes of the creation, sustenance and destruction of the manifested universes. He is directly and indirectly conscious of all manifestations, and He is independent because there is no other cause beyond Him. It is He only who first imparted the Vedic knowledge unto the heart of Brahmājī, the original living being. By Him even the great sages and demigods are placed into illusion, as one is bewildered by the illusory representations of water seen in fire, or land seen on water. Only because of Him do the material universes, temporarily manifested by the reactions of the three modes of nature, appear factual, although they are unreal. I therefore meditate upon Him, Lord Śrī Kṛṣṇa, who is eternally existent in the transcendental abode, which is forever free from the illusory representations of the material world. I meditate upon Him, for He is the Absolute Truth.
I discuss this verse in my question here, by the way.
On a side note, it's great that you're watching Shri Krishna. I watched that show when I was in Kindergarten, and it was one of the main shows that got me interested in learning more about Hinduism. It led me to read Amar Chitra Katha comic books, which greatly increased my knowledge of Hindu stories and ultimately led me to start reading Hindu scripture.