Movable do

In the movable-do system, the syllable 'do' (d / 1) shifts to whatever pitch is the tonic of the music's key. In C major, do = C; in G major, do = G; in B♭ major, do = B♭. The relationship between syllables stays constant, do to mi is always a major third, sol to do is always a perfect fourth, which lets singers transpose and sight-read by interval, not by reading absolute pitches. This is the opposite of fixed-do solfège, where do is always C regardless of the key. Movable do is the system used in tonic solfa, jianpu, and the Curwen/Kodály pedagogical traditions.

In context

Example	What it means
Key: C, do = C, mi = E, sol = G	In C major the tonic chord (do-mi-sol) sounds as C-E-G
Key: G, do = G, mi = B, sol = D	Same syllables (do-mi-sol) now sound as G-B-D, same chord function, different pitches
A melody reading d m s d' in any key	Always outlines the tonic major triad and the octave above

Sources

Reference: en.wikipedia.org , consulted for the definition above; DomiSol's wording is original.