If you’re building a circuit, you need some form of power, usually a battery. Think of a battery like your normal everyday food. The stuff you’d normally eat throughout the day.
A capacitor is more like an energy drink. If a part in your circuit needs a sudden burst of energy (like a motor that needs to suddenly move), you can use a capacitor in the circuit so it can get that energy when it needs it. Just like you’d drink an energy drink when you need a sudden burst of energy.
To help you remember, capacitors can even LOOK like little cans of energy drink!