A ship's reactor provides power to essential ship systems, but many modules in order to be activated require a burst of power which the reactor is unable to provide. Capacitors therefore are installed to meet this need. The reactor fills the capacitor with power, the capacitor stores this power until modules are activated. As modules are activated, power is drained from the capacitor, which then causes the reactor to add more power.
A capacitor has two attributes: capacity (how much power it can store) and recharge time (how much time is needed for the reactor to fill it).