a capacitor
Picture a capacitor as a tiny rechargeable bucket for electric charge. Two metal plates face each other, separated by a thin layer of insulator. Like a small stretchy water tank that fills and empties, it stores charge and resists sudden changes in voltage. If voltage is water pressure, a capacitor is a little tank that smooths pressure changes.
Here is how it actually works. Push charge onto one plate and it repels charge off the other, building up a voltage across the gap. The defining rule is that the current flowing into a capacitor equals its capacitance times how fast the voltage changes: I = C times dV/dt. So if you force 1 mA into a 1 microfarad (uF) capacitor, its voltage ramps at dV/dt = I/C = 0.001/0.000001 = 1000 volts per second. Once the voltage stops changing, the current stops too, so a capacitor blocks steady DC but passes changing signals.
Capacitors are everywhere: smoothing power supplies, coupling audio between stages while blocking DC offsets, setting timing in oscillators, and decoupling chips so a sudden current demand does not sag the supply. Honest caveat: a real capacitor is not ideal. It has leakage, a little series resistance (ESR) and series inductance, and electrolytic types have polarity and wear out. The flat blocks-DC, passes-AC picture is a starting model, not the whole truth.
A 470 uF capacitor across a 5 V supply rail acts as a local energy reservoir: when a chip suddenly draws extra current, the capacitor supplies it instantly while the slower main supply catches up, keeping the rail from dipping.
A decoupling capacitor smooths sudden current demands.
A capacitor does not store current. In steady DC there is no current through it at all; it stores charge and energy in the electric field between its plates.