What does this symbol refer to?

What is the symbol that is below the C1 capacitor and R1 resistor. Is it the symbol of grounding?

Answer

The symbol represents signal ground. This ground is often connected to the usual ground of a device, commonly known as "earth". However if a system is very sensitive the two grounds cannot simply be connected, but require a thorough engeneering approach and a good ground able to handle high currents.

More info on the grounding types:

https://en.wikipedia.org/wiki/Ground_(electricity)#Electronics