All Tkinter widgets have a border (though it's not visible by default for some widgets). The border consists of an optional 3D relief, and a focus highlight region.
The relief settings control how to draw the widget border:
borderwidth (or bd) is the width of the border, in pixels. Most widgets have a default borderwidth of one or two pixels. There's hardly any reason to make the border wider than that.
relief controls how to draw the 3D border. It can be set to one of SUNKEN, RAISED, GROOVE, RIDGE, and FLAT.
The highlight settings control how to indicate that the widget (or one of its children) has keyboard focus. In most cases, the highlight region is a border outside the relief. The following options control how this extra border is drawn:
The highlightcolor is used to draw the highlight region when the widget has keyboard focus. It's usually black, or some other distinct contrast color.
The highlightbackground is used to draw the highlight region when the widget doesn't have focus. It's usually same as the widget background.
The highlightthickness option is the width of the highlight region, in pixels. It is usually one or two pixels for widgets that can take keyboard focus.