Drop shadow effect
Add a drop shadow under your object's visible on the layer.

The distance and rotation properties set how far the shadow is offset and in which direction, while blur controls how soft its edges are. You can also change the shadow's color and its alpha (transparency). Enable Shadow only to hide the object itself and keep only its shadow.
Note
This is a 2D effect. It has no effect on 3D objects or 3D layers.
Reference
All effects are listed in the effects reference page.