The Masking Brush is one of two tools used for masking filters in Effects, as well as layer masks. When you select the Masking Brush, your cursor changes to look like two concentric circles, one representing the inner hard edge of the brush, and soft outer, feathered edge of the brush. At the center of the brush is either a plus or minus. The plus means that you are painting “in” a filter’s effect, while minus means the brush mode is set to paint out.