The RC time constant is the product of the resistance and capacitance of a circuit, or R x C. It is used to describe the rate of charging when a battery is connected to a resistor and capacitor in series. RC circuits can effectively function as timers.
When the RC circuit is closed, the battery charge starts to move from one plate of the capacitor to the other, at which point the initial current is high. As the charge in the capacitor increases, the charging current decreases. The capacitor stores its charge in the electric field between its two plates.