A capacitor is an electronic component, which is in affect an electronic equivalent of a water tank with two inlets and a movable diaphragm in the middle. It can be charged until it is all in one direction, or charged the other way - but once charged, there is no more water flow, and so it must be discharged…
That is it will in-effect store up voltage for later. It is also used for smoothing out ripples and spikes in currents.
The most basic capacitor would be two metal plates- opposite each other. Each plate is capable of building up a positive, and negative charge. This charge is proportional to the size of the plates(in fact the size of the overlap of the plates as well), and the amount of voltage being put through to store the current. Eventually they will become fully charged - and will no longer take any more current.
If you then connect them to an output device, they will output a current until they are fully discharged. Because with a certain voltage, resistance and capacitance you will get a fixed repeatable time - a great application for these are timing circuits.
They are also good for filtering out low frequency when used in series, or for filtering high-frequencies when connected in parallel to ground. By using a combination of capacitors and inductors you can create a tuned filter which suppresses all but a limited range of frequencies.