Metacity is a window manager for Gnome. In Layman's terms it decorates windows with themes, icons.
The Metacity "composite" is a rendering layer that adds some effects to make the office aspect more enjoyable.
It is significantly less advanced than Compiz, defacto making it much lighter than the latter.
Activating this feature allows for:
True transparency, for example:
the terminal with transparent background.
Gnome-do, which then activates its "glass" theme.
notify-osd that displays in blur.
AWN: this "dock" requires either Compiz or Composite to work.
A shadow effect on the windows, it's quite pretty.
If you have a computer that can't handle Compiz, it may be a good alternative. You can activate it under Gnome with this command: