Welcome to the site gdaryl333. Building an EV is a great way to get away from the gas station!
To answer your question, Cd is coefficient of drag...
"The drag coefficient (Cd, Cx or Cw, depending on the country) is a dimensionless quantity that describes a characteristic amount of aerodynamic drag caused by fluid flow, used in the drag equation."
http://en.wikipedia.org/wiki/Drag_coefficient