What is signal ground
Figure 1. Various earth and ground symbols. Source: Ground, earth, chassis.

“Ground” is a reference point in an electrical circuit. It is used as a reference point for voltage measurements. As a result a voltage may be above ground (positive) or below ground (negative). This is very like a surveyor taking a reference point in a certain location and referencing all other points to that datum.

The most common reference is Earth itself. Power systems are usually “earthed” at some point to provide a reference for the system voltages. The earth symbol represents the parallel plates that were buried in the soil to ensure good conductivity. (The plates were connected by wire and early forms of the symbol show the vertical line connecting all the plates. The modern “cleaner” symbol omits the vertical.)

The ground symbols indicate the generic reference point. Even if there is no earth or chassis connection it is common to refer to one point or voltage in the circuit as “ground”. In equipment where electrical isolation is provided between sections of the circuit two or more ground symbols may be required to indicate which ground the components are connected to.

I have written further on the topic and given circuit examples in the linked article.