The gamut; a traditional way to sing the octave. "Do" refers to C, "re" to D, etc.
The syllables are derived from a Latin hymn:
Ut queant laxis resonare fibris
Mira gestorum famuli tuorum,
Solve polluti labii reatum,
Translated by dann:
"Ut queant laxis" = "just as your servants"
"Resonare fibris" = "may sing freely"
"Mira gestorum" = "of the miracles"
"Famuli tuorum" = "of your works,"
"Solve poluti" = "release the stains (of sin)"
"Labii reatum" = "from their lips"
"Sancte Johannes" = "Saint John"
"Ut" has been "do" for a long time, but it's still the gamut and not the gammado. "Sol" is still common, but I don't know what happened to si.
A more famous appearance of the gamut is in The Sound of Music, with a song that goes something like:
Doe, a deer, a female deer
Ray, a drop of golden sun
Me, a name I call myself
Far, a long long way to run
Sew, a needle pulling thread
La, a note to follow So
Tea, a drink with jam and bread
That will take us back to... (repeat)