7. The Menus / 7.8 The Effects Menu
Lightning creates a lightning. This effect filter can be used without an active image layer. As a result you will get a group containing vector layers.
The subdialog Lightnings controls the basic look of the lightning: its Position, the Width and the Color.
Parameters defines the exact appearance of the lightning. The settings are quite abstract, therefore you should "play" a bit with them in order to get a feeling for their effects.
Glow and Light control the appearance of the shine of the lightning: the Width, the Intensity and the Color.