The rectangle tool allows you to draw a rectangle. Simply click and hold down the mouse, drag until you get the size rectangle you want, then release the mouse button.
The stroke width option allows you to specify how thick of a border you want for the rectangle, while the current Stroke Color will be used for the color of the rectangle's border.
Elements drawn with the Rectangle tool can be filled with either a solid color or a gradient. You can also apply a drop shadow to rectangle elements, as well as bevel/emboss them, or add a glow to it.