Capacitor actual value with DC bias
What is a DC bias
DC bias is a measurement of the average voltage on a wire on which can &quot;ride&quot; a signal, should this wire transmit data, and noise.
Its unit is DC Volts, or VDC. You can remember it because it is the direct voltage on the conductor.
Variation of stored charge as  a function of DC bias
Onto the phenomenon you describe in your question : as the signal varies, the capacitance &quot;seen&quot; by it will vary, introducing nonlinearities. But, the DC bias being most of the time large compared to the signal (or noise) voltage, they will be small.
Thus, you can use the DC bias as an effective voltage with which to consider the effective capacitance of your component, if and only if the voltage across the capacitor does not vary too much.
For example, if a \$5\$ to \$-5\$ volts audio signal had a \$7\$ volts DC bias and one end of the capacitor was grounded, the overall voltage would range from \$2\$ volts to \$12\$, which is a huge variation. You should not rely on a time constant or a precise capacitance then. If it was a \$\pm 200 \text{ mV}\$ signal on the other hand, you most likely could.