In movable-do solfège, ド is the first note of any major scale. Common in Japanese music education.
from Italian
In C major, do is the note C.
In fixed-do solfège, ド always refers to the note C, regardless of key.
This melody starts on do.
In fixed-do, do always refers to the note C.
ピアノでドを弾いてみて。
Try playing do on the piano.
ド is the first note (do/C) in solfège, while ソ is the fifth note (so/G); learners often mix up the order of these syllables.
Borrowed from Italian 'do', the first note of the solfège scale.