The Solfeggio frequencies are a set of frequencies derived from the Solfege musical scale. From Wikipedia:

In music, solfège, also called sol-fa, solfa, solfeo, among many names, is a music education method used to teach aural skills, pitch and sight-reading of Western music. Solfège is a form of solmization, though the two terms are sometimes used interchangeably.

Italian “solfeggio” and English/French “solfège” derive from the names of two of the syllables used: sol and fa.

In eleventh-century Italy, the music theorist Guido of Arezzo invented a notational system that named the six notes of the hexachord after the first syllable of each line of the Latin hymn “Ut queant laxis“, the “Hymn to St. John the Baptist“, yielding ut, re, mi, fa, sol, la.

https://en.wikipedia.org/wiki/Solf%C3%A8ge

These frequencies have been associated with various healing properties, and there are several examples of these frequencies being researched in scientific studies, as well as sound in general.

• 528hz – Effect of 528 Hz Music on the Endocrine System and Autonomic Nervous System