The gradient allows you to create a region of softness around the mask edge. It applies a gradient according to the distance of the borders from the mask spline. You can create a uniform gradient or customize it at different parts of the mask. The gradient can have two softness borders, one inside and one outside the mask edge.
The position and distance of a gradient point from the mask spline can be controlled gesturally in Move mode (M) or in the Gradient tab.