When you make an AC analysis the nodes that are connected to fixed (DC) voltages (like supplies) do not change (be definition). So, from the AC point of view, you can connect all those nodes together in only one node - usually called "AC ground".
The "virtual ground" is a node that is NOT connected to a fixed voltage, but whose voltage does not vary when you apply a signal to the circuit.
An example is the simple inverting amplifier made with an opamp and two resistors: the "+" input of the opamp is connected to a fixed voltage (usually gnd). The "-" input of the opamp is connected to the two resistors where the signal current flows. However, due to the high gain of the opamp, the "-" and "+" terminals have always similar voltages -> Therefore if the "+" terminal is connected to GND, the "-" terminal has also 0 V potential.
Thus, in this case, the node where the "-" terminal connects is called the "virtual gnd" because the node is not physically connected to a fixed voltage, but its voltage does not vary. To analyze the circuit you can then consider that the node is really connected to gnd.
Regards