A Rectangle line allows you to draw an Rectangle on your chart.
To draw a Rectangle line select it from the toolbar. Once it is selected left click the first point on the chart then left click the second point on the chart and the Rectangle will be drawn.
Editing Lines
Once a line has been drawn on the Chart, right click on the edge line and select Line Study Properties.
To Move a Rectangle, left click once on the edge line to select it, then you can left click to drag it anywhere on the screen.
To re size an Rectangle, left click once on the edge line to select and then left click again on the anchor points to re size.
Right clicking on an edge line will allow you to change the properties of that drawn line. You can globally change the default setting for the Rectangle by right clicking on the Rectangle icon on the toolbar.