Why Alternating current periodically reverses its direction of flow, moving first one way through the circuit and then the other. In household AC and in RF signals the voltage swings above and below zero, so the current flows positive, then negative, repeating at some frequency (60 Hz for US mains). Direct current, by contrast, always flows in one direction even if its amplitude varies.
Watch out The choices about swinging between one polarity and zero describe pulsating DC, such as the output of a half-wave rectifier: the amplitude changes but the direction never reverses, so it is not truly alternating.
Alternating means the direction actually flips: plus to minus and back. Just varying in size without reversing is still DC.