What do the Tire Size Numbers mean?

205/55 R16 91W

The numbers in the tire size refers to different parts of the tire.

The first number (205) identifies the section width, or the cross section, in millimeters.

Typically following the section width is the sidewall aspect ratio or profile (55). The sidewall height, from rim to tread, is 55 % of the section width of 205, or 112.75 millimeters (4.44 inches).

The last part is the internal construction of the tire. R stands for radial constuction, in which the tire's body plies "radiate" out from the imaginary center of the wheel. Virtually all tires sold today are of radial construction.
The number following (16) indicates which wheel diameter this tire is designed to match. Tires have rim diameters expressed in inches.

The 91W at the end of the tire size is the tire's service description. A service description tells you what load index and speed rating a tire has.