Press the top push button S1 and hold it down for about half a minute. Release that button then press the other push button S2 and hold it down. The LED will glow brightly then fade away quickly to nothing. The LED will be on for about a half a second. Repeat this a few times. Notice that the LED is initially as bright as when it is connected to 5 volt supply via the 1k resistor.
What happens is this. When push button S1 is pressed the 5 volt supply is connected to the capacitor via the 100k resistor and current flows into the capacitor and gradually charges it up to almost the full 5 volts. When S1 is released the capacitor still has 5 volts across its terminals. At the moment when push button S2 is pressed the charged capacitor behaves as if it is a 5 volt supply but as current flows out of it and into the LED the charge is dissipated and the voltage at the terminals of the capacitor rapidly drops to below 2 volts.
What you have learnt by this is that a capacitor is a device which will store electrostatic energy.