What is the symbol that is below the C1 capacitor and R1 resistor. Is it the symbol of grounding?
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:
It means Equipotential level. Often used, in CAD, for analog ground or signal ground. See IEEE STANDARD 315-1975 - "IEEE Standard for Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters)"
Yes it is. Gound or zero potential depending on your application.