A Capacitor stores electric charge. Basically a capacitor consists of two metal plates separated by an insulating material called “dielectric”. When connected to a power source such as battery, the capacitor charges up until the potential difference between its terminals become equal to the e.m.f of the battery.
When a capacitor and a resistor are connected to AC lines, a constant current can be maintained through the resistor, so long as the Rectance of the capacitor is greater than the resistance of the resistor. The current flow depends on the value of the capacitor assuming that the input voltage is greater than the output voltage.