"Putting a capacitor into your circuit is like building an extra reservoir - a water tower, for example - to capture the flow. You can then let the flow out of your water tower (capacitor) at a very steady rate, or all at once, in a burst. This extra reservoir can be useful in a couple of different ways. For example, it might be important that the machine being powered by your current (whether a water wheel or light bulb) receives a very constant current that is not too big or too small. Again, too big might hurt the machine; too small and it will stop working."