The description of a tire size, in general, is formed by the nominal measurements of the section width and the tire's inner diameter, and may include the nominal aspect ratio (series) between the two measurements.
Example:
205/55R16
205 - Indicates the nominal width of the tire expressed in millimeters.
55 - Indicates the percentage quotient between the height of the section and the nominal width of the tire approximately equal to 55.
16 - Indicates the inner diameter of the tire expressed in inches.
R - The letter "R" indicates the type of construction "Radial".