Question

What is a virtual ground? I would like to know what it is

Answer 1

"Ground" refers to a particular voltage, generally taken to be "zero", or the voltage of the earth. A "virtual ground" is a wire in a circuit whose voltage is held to be zero not because it is directly connected to the true ground, but instead because it is actively driven to that voltage typically by feedback mechanisms.

Here's an example of a virtual ground circuit:
Virtual ground circuit
In the above, the large triangle represents an OP-AMP, that is, an operational amplifier. It drives its output (marked VGND) to be + voltage when its + input has a higher voltage than its - input, and to be - voltage when its - input has a higher voltage than its + input. Since the + input is connected to true ground (marked GND), the op-amp will keep it output VGND at the GND level (to the best of its ability). Since the VGND wire is held at ground but is not connected to ground, it is called a "virtual ground".

By the way, in the usual "electric-circuits" limit, circuits are composed of "wires" where we make the assumption that the voltage of a wire is the same no matter where on the wire we look. That is, we ignore the process by which a signal is sent down a wire