Match a target background color

Anti-aliasing makes an object appear smoother by blending its color into the background. For example, if the object is black and the page on which it rests is white, anti-aliasing adds several shades of gray to the pixels surrounding the object's border to make a smoother transition between the black and white.

 In the Optimize panel, select a color from the Matte pop-up menu. Match it as closely as possible to the target background color on which the graphic will be placed.

 community help (adobe.com)