Mask painter is used for marking a particular region of the terrain to protect it from editing, to enable Mask painter, click on the Mask tab in the Inspector or press F2.
Under the General foldout, there are some settings for the brush, such as:
• Id: Id of the brush, click on its label to ping the brush asset in Project window.
• Radius: Brush radius in world space.
• Always Show Mask: Check this to display masked region when working with other painters.
You can clear to invert the masked region by using the button under Data foldout.