A Time Ripple is a disturbance in time, creating a portal that allows Time Travel. Whenever something stops a destined event from occurring, it sends a ripple through time. A powerful being who knows the place where destiny was cheated can use a Time Ripple to travel back and forth through time and alter the course of history.
One such Ripple opened when Phoebe prevented Miles's fated death. The warlock Bacarra used this moment to travel from the future to the present. Piper later used the same Time Ripple to reverse the day in order to save her sisters.