"DO" a deer, a female deer... The song made famous in the musical "The Sound of Music" was based on Solfege. Solfege is the "DO-RE-MI" that most people are already familiar with, but for the musician Solfege gives us a tool to use that is the same in every key.
Solfege remains constant to scale degrees regardless of the key. What I mean by that is, "DO-RE-MI" is always the 1st, 2nd and 3rd notes of the Major Scale. In "C" Major, DO=C, RE=D, & MI=E. The 1st, 2nd and 3rd notes of the "C" Major scale. In "F" Major, DO=F, RE=G & MI=A. Again the 1st, 2nd and 3rd notes of the "F" Major scale.
Solfege therefore provides a way to look at music and melody that is constant across all keys. Solfege helps us transpose (put music in another key). An alternative way to look at Solfege is by number of scale degree. Meaning DO=1, RE=2 and so on. Some find this approach to be more natural.
Solfege Chart: Shown as key of "C" with notes in black from the Chromatic Scale.