Drop shadow effect
Add a drop shadow under the objects visible on the layer.

The position of the shadow is set with Distance (how far the shadow is offset) and Rotation (the direction the shadow is cast). Use Blur to soften its edges, and Color and Alpha to change its tint and transparency.
Enable Shadow only to render just the shadow (the object itself becomes invisible). This is handy to draw a shadow as a separate object that you can place and move independently.
Reference
All effects are listed in the effects reference page.