Specify a start point and end point for a line, or specify the line length.
A radius creates fillets at the specified radius between line segments, allowing you to create a rounded chain of lines. A line must be joined to the end of another line for a fillet to display. If a line is connected to more than one line, a fillet cannot be created.
The following image shows line segments sketched without Radius selected and line segments sketched with Radius selected.