Signal Ground (SG) is the ground reference, not the ground potential, and thus the reference point for the signal. Due to the unavoidable voltage drop when current flows, a circuit can have different ground potentials which differ slightly in level.
Level-wise different ground potentials lead to the fact that potential shifts can occur at amplifier circuits when the ground or shielding is connected with the signal ground, which become noticeable as disturbances or in the form of hum loops. For interfaces and connectors, the signal ground forms the reference potential of the signal voltage. The signal ground can be completely independent of ground references of other electronic circuits or devices, but it can also be connected to the ground of the supply voltage.
The signal ground is sensitive to interference and can pick up noise. In audio circuits, it is therefore important to ensure that the signal ground is shielded and the ground is kept separate by circuitry.