The Baal Shem Tov (Rabbi Israel) was known for his love of the simple, heartfelt service of God. He taught that music and song can break through the gates of heaven, even when words cannot.
Based on the phrase "tova_e_edna_ot_nai_hubavite_pesni_za_boga_ot_br..." (Bulgarian for "This is one of the most beautiful songs for God by Br..."), this likely refers to a story or melody associated with the (often abbreviated as Br... in Bulgarian transliteration or referencing a "Brother" or "Brotherhood" context), a historical figure who often taught through stories and joyful melodies.
While the partial quote could refer to a specific niggun (melody) passed down, it is fundamentally tied to the Baal Shem Tov's philosophy that a simple, beautiful song is one of the most powerful prayers to God. Listening to the Dark: The Descent of the Goddess Inanna