What is the meaning of Sol-fa?

a method of sight singing music that uses the syllables do (originally ut), re, mi, fa, sol (or so), la, and si (or ti) to represent the pitches of the scale, most commonly the major scale. The fixed-do system uses do for C, and the movable-do system uses do for whatever key the melody uses (thus B is do if the piece is in the key of B).

To sing a sol-fa

Source: wiktionary.org