In alternating current (a.c.) the movement of charge changes direction. The charge moves forwards, then backwards, then forwards again, over and over. Because the voltage is reversing, the current keeps changing direction too.
This is different from direct current, where the charge keeps moving one way round the circuit. In an a.c. circuit the charges move backwards and forwards about a fixed position, so they do not travel steadily round the circuit in one direction.
The number of complete forwards-and-backwards cycles each second is the frequency, measured in hertz (Hz). A lamp glows when the current flows either way round, so a lamp works with a.c. and with direct current. The mains supply is alternating current.