Menu: View→Interface Properties→Accessibility
A focus rectangle is a visible rectangle in a window or dialog box. The focus rectangle encloses the portion of the window or dialog box that receives keystrokes that you enter at the keyboard.
Sometimes, Windows automatically draws a focus rectangle to show where your keystrokes are directed. When a focus rectangle would be a helpful cue and Windows does not draw one automatically, Amos draws a focus rectangle. For focus rectangles that Amos draws, you can control the thickness of the rectangle's border by specifying a value for View→Interface Properties→Accessibility→Focus Rectangle Thickness.
If you do not find the focus rectangles to be helpful, you can eliminate the ones that Amos draws by setting the focus rectangle thickness to zero.