Capacitors store energy from another power supply. Charging through an LED, you will see that the capacitor charges pretty quickly. The stored charge in the capacitor provides the power to light the discharge LED, which also happens quickly.
Video below shows an easy way to build this circuit on an electronics prototype breadboard.
Affiliate link ad. Americans using these links supports the site at no extra cost to you!
This circuit works better with a higher voltage supply since both LEDs drop voltage from the capacitor.
As an Amazon Associate I earn from qualifying purchases.