Draw Mode turns the mouse into a drawing cursor. You can select the type of drawing you want to do (i.e. lines, circles, infinite lines) by toggling through the drawing types. You can toggle by either pressing the spacebar or continuing to press F6. With the mouse in Draw mode, the C key will toggle the drawing color, and the V key will toggle the line width.
Width
The line width selection toolbar pops up from the main menu when Draw/Width is selected. It also appears when selecting a width in the Path dialog box.