A motion guide is a guide layer that has an adjacent layer below it set to guided. A motion tween in the Guided layer will follow any path drawn in the Motion Guide layer. That means you can draw an s-shaped line in a Motion Guide, and then the Guide layer can include a Motion Tween that follows the shape. A Motion Guide will be invisible to the user when the files is published or the guide layer is hidden.

ex.

download the motion guide tutorial (file name: 5motgtut.fla)
right click on the file (control-click for Mac) and select to download to your drive.