This filter adds a new layer with some clouds to the image that look like fog or smoke. The clouds are created with the Plasma texture.
Among the few filter options, only “Turbulence” is somewhat important, because you can't change it later and have to undo and repeat the filter if the result doesn't fit your desire.
The name of the layer. You can change it later in the Layers Dialog.
Defaults to some kind of sandy brown (240, 180, 70). Click on the color button to change this if you think that is not the natural color of fog.
This is actually the Turbulence option of the Plasma filter: it controls the complexity of the clouds, from soft (low values) to hard (high values).
The opacity of the layer. You can change it later in the Layers Dialog.