Why A mobile whip on HF is far shorter than a quarter wave, so it looks capacitive and needs loading to become resonant. A capacitance hat is a set of spokes, a disc or a wheel added near the top that adds capacitance to ground, which shifts the current distribution up the whip and makes the antenna behave electrically like a longer radiator. That lets you reach resonance with less loading coil inductance, and the lower coil loss usually raises efficiency.
Watch out The choice about reducing radiation resistance has it backwards: by keeping more current flowing higher on the whip, a capacitance hat tends to raise radiation resistance, which improves the ratio of radiated power to loss.
Hat on top makes a short whip 'taller' electrically. Cap hat = electrical length, not power rating.