The letters you have stated are the first letters of each of the solfege note names in music: Do, Re, Mi, Fa, Sol. The next letter is L for La.
Solfège is a system for singing notes. If you’re familiar with the famous Rogers and Hammerstein song “Do-Re-Mi” from The Sound of Music, you already know the solfège note names: do, re, mi, fa, sol, la and ti.
The original solfège—also called solfeggio and solmization—note names derive from an 11th-century hymn by Guido d’Arezzo, in which the solfège syllable is the first note of each phrase. The starting notes of each phrase are C, D, E, F, G, A.
Latin-derived languages (Italian, French, Spanish) assumed these names, and ut was eventually changed to do in Italy and later in the other countries. The solfège syllables are thus nothing more than the note names in French, Italian and Spanish, minus any accidentals.
Of interest, why are Notes of the Tonal Scale Called "Do, Re, Mi"? visit Mentalfloss.com to know.