What does the label on a tire mean?
The 40 is the inflated width in mm and the 622 is the internal diameter in mm. It's the international standard ISO-5775 for tire sizes. In brackets are the French size (700mm outer diameter x 38mm width) and the Inch size (28 outer diameter x 1 5/8 height x 1 1/2 width)

Wikipedia has a very detailed explanation of tire sizes and designations, I confirmed my answer on this page.