Why Reactive power is the energy that sloshes back and forth between the source and a pure reactance (inductor or capacitor) without being consumed. In an ideal inductor or capacitor the current leads or lags the voltage by exactly 90 degrees, so the average of the instantaneous power product over a cycle is zero, meaning no net energy is dissipated. Real (dissipated) power, by contrast, occurs when voltage and current are in phase, as in a resistor; in general P = E * I * cos(theta), and at theta = 90 degrees cos(theta) = 0.
Watch out The vague choice saying simply "out of phase" is tempting, but any phase angle other than 0 gives some reactive component; the pool wants the specific 90 degree case that is purely reactive with zero real power.
Reactive = 90 degrees (cos 90 = 0, no real watts). Real power = 0 degrees, in phase.