The Drop Shadow option lets you add a drop shadow to the selected element. Once you've added the shadow, the Drop Shadow options panel will appear to allow you to customize the shadow.
The Edge Softness option controls how blurry the edges of the shadow will appear (higher values will appear blurrier). The Shadow Size option controls how big the shadow is relative to the element that is casting it (positive values are larger than the base element, negative values are smaller than the base element). The horizontal and vertical offset options allow you to position the shadow where you want it.