You can create interesting effects by making objects more opaque or less opaque.
When you put a low-opacity element on top of another element, for example, the bottom element shows through the top element. Depending on how high or low you set the opacity, the elements below can be highly visible, partly obscured, or completely blocked from view (at 100-percent opacity).
Elements can have a separate opacity on their fill color. See Filling Elements with Colors or Images for more information.