Appearance parameters define how the shapes will visually look. There are three styles and all of then can be applied to a shape.
This is the main color of the image. It affects the entirety of the image. It can have alpha.
Modify the stroke size to create a stroke of your shape. Stroke width of 0 means no stroke. Stroke uses the main color.
Falloff creates a fade out effect along the edges of the shape. It can be used to make the shape fuzzy and can help reduce aliasing of sharp edges or just be a cool effect. It can be used to create soft shadows under an image.
Outline can be applied with or without stroke. Outline width 0 means no outline. Main Color will not affect the outline and so it has a seperate color field.