When used as an option to the first cell in a sequence of selected cells, AnimationDirection specifies the direction in which to run animations involving those cells.
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->ForwardBackward specifies that the animation should run from the first cell to the last, and should then reverse back to the first cell again.