This example will help you understand motion path more clearly :-
Animate PowerPoint Graphics using Motion Paths
PowerPoint 2007 - On the Animations tab of the ribbon click the Custom Animation button. Add the Custom Motion Path
In this example, we will create a meandering path for the graphic object to follow on the PowerPoint slide.
For selecting the graphic object
In the Custom Animation, select the following -
Add Effect > Motion Paths > Draw Custom Path > Scribble Meandering Motion Path
Using the Scribble option for a motion path allows you to draw any type of meandering path for the graphic object to follow.
Place the mouse pointer next to the graphic object and draw a line on the slide where you would like the object to follow.
Make Modifications to the Motion Path
Once the motion path has been drawn on the slide, you may wish to make modifications to the speed or whether the animation is applied on click or automatically. These options can be changed in the Custom Animation task pane.
Test the Motion Path Animation
At the bottom of the custom animation task pane, click on the Play button to see the motion path animation applied to the graphic object.
If you do not like the result, you can simply select the motion path and press the Delete key on the keyboard to remove it. Repeat the previous steps to draw a new motion path.