The Mask tool lets you add mask to the object. Select the mask tool from the Tool Panel, 

Create Mask:

Position the pointer where you want the mask curve to begin, and hold down the mouse button. The first anchor point appears. Drag in the direction you want the curve segment to be draw. and release the mouse button once you have positioned the first direction point. The length and slope of the direction line determine the shape of the curve segment. You can adjust one or both sides of the direction line later .then Do one of the following:

• To draw the next segment of a smooth curve, position the pointer where you want the next segment to end, and drag away from the curve.
• Double click  mouse button to break out. 

Edit Mask:

   check modify opotion in the option page, modify mask curve control point