That is the visual indicator mask that LR gives you to see where your brush strokes have been made. To turn the mask on or off, press the letter "O" (not zero).
Here's my method of using the brush tool: I open the brush tool and with ALL settings at their default (double-click each control knob to reset it), move my mouse cursor to the area I want to adjust and click the mouse on that spot. I have a dot with a target dot inside it. This is my active brush area.
Now I press "O" and turn on the mask and start brushing where I want to make a change. A pink aura appears. Now I hold the mouse button down (you'll see a "+" sign in your cursor) and brush in the area until I've got pink covering the entire location. If I've overshot the runway and exceeded the edge of something, I click on the Erase button (brush turns to a "-" sign) in the brush tool area and "erase" the overrun.
Press "O" to turn the mask off, and go to your brush adjustment dials and start adjusting. It's magic! Click Done when you are finished.
Each time you create a brush area you'll see a gray dot. The one with the black target in the middle is the active area.
Hope this helps!
