AnimationDirection is an option for Cell which specifies the direction to run an animation which starts with the cell.
The setting AnimationDirection->Forward specifies that the animation should run through successive selected graphics cells in the order that they appear in the notebook, and should then start again at the first cell.
AnimationDirection->Backward specifies that the reverse order should be used.
AnimationDirection->ForwardBackward specifies that the animation should run from the first cell to the last, and should then reverse back to the first cell again.
It is the setting of AnimationDirection for the first graphics cell in the sequence selected that determines the animation direction for the whole sequence.
See The Mathematica Book: Section 2.10.12.
See also: AnimationDisplayTime.