EASA / CAR Part-66 · Module 4 · 4.1.1 Diodes · Cat B1/B2
A rectifier gives bumpy DC. A smoothing capacitor stores charge and fills the gaps between peaks. Increase the capacitance (or lighten the load) and watch the ripple shrink toward smooth DC.
Between peaks the capacitor discharges through the load, so the voltage sags, that sag is the ripple. A bigger capacitor (or smaller load current) holds the voltage up better, giving smoother DC. Ripple ≈ load current ÷ (capacitance × frequency).