An empty leg is a flight an operator is flying anyway, often to bring an aircraft home or to its next trip. Because the aircraft is going regardless, the operator sells that flight for less. You book the whole aircraft, not a seat.
Empty legs move when the main trip moves. If the operator's schedule changes, your leg's time can change or the leg can be cancelled, so they suit flexible plans best.
Fixed-price one-ways are different: the operator sets a price on a route and keeps it open on the dates it chooses. Watch a route on the empty legs page and we'll email you when a leg posts on it.