Follow path is a constraint which creates an animation in which the Object follows a Curve. You of course have to constraint it to a curve of some kind.
When you added a curve you've to click the "Animate Path" button, otherwise nothing will happen.
The "Follow Curve" checkbox is there to make the objects Forward Axis, here "Y", point in the direction of movement.
Of course here you'll find the Forward axis choices and Up choice, weird they still got different layouts, but don't bother.
Finally, the "Influence" slider.