In a mathematical description of a circuit, C stands for capacitance. (Charge is denoted by the letter Q, current is denoted by the letter I, and conductance is denoted by the letter G)

Capacitance is simply a number that relates the voltage across a capacitor to the amount of charge on its plates.

A capacitor is a device that stores energy in the form of electric charge. It consists of two metal plates separated by a short distance or a dielectric substance. If an external voltage is applied across the two plates, current flows such that one of the plates collects positive charge and the other plate collects negative charge. Current continues to flow until the amount of opposing charge held in the two plates produces an internal voltage that exactly counteracts the externally applied voltage.

A capacitor acts something like a short-term rechargeable battery, charging up when an external voltage is applied to it and discharging when the external voltage is removed.