A "treatment" is a document that lays out a director's vision for a commercial. It's used as a roadmap for the director and crew - and as a way of communicating the unique vision to the client. It typically includes at least the script and a storyboard, but it can contain anything else that helps the director envision the spot before he or she begins shooting: casting notes, visual tone, location inspiration, scouting photos, and much more.
Treatments are invaluable tools for prepping shoots because they allow directors to work out hundreds of details before shoot day. The result is typically a better-prepared and less-stressful shoot, plus a more professional and better executed finished product.