Just like the "Draw Point" radio button, except that the points are connected by a line. To draw a line:
Select the "Draw Line" tool, move your cursor to the place on the map where you want the line to start, and click down once. Now, when you move the mouse, a line will connect the cursor to the location of the first point.
Click again to lay a second point. Keep on clicking to add multiple points. You can also draw a smooth line that automatically follows the movement of the cursor. Hold down the Shift key and then click and drag on the map. This lays down a new point for each individual pixel movement registered by the browser.
Tip: This feature has the effect of creating extremely dense geometry, which is very difficult to maintain (eg, if you decide later on that you want to change the shape of the line, you would have to manually move hundrds or thousands of individual points). If you need smooth, curvy shapes, always see if you can use the "Draw SVG" feature, which makes it possible to import easily-maintained vector graphics created in programs like Adobe Illustrator and Inkscape.
When you're ready to lay down the final point, move your cursor to the place where you want the line to end and double click. This "completes" the line.