When you perform secondary colour grading, whether with a geometry, a key, or an external matte, it is important to see what part of the image is being affected by the matte. The Matte Overlay feature displays the matte in its original colour, while overlaying the unselected region of the matte with a uniform colour, allowing you to see the precise area of your image that will be colour corrected.
You can modify the Matte Overlay settings at any time when you are performing secondary colour grading without affecting the image output. See Adjusting the Colour and Opacity of a Matte Overlay.