Tyre width is the first of the values determining the size of the tyres in a car, it is given in millimetres and determines the distance between the tyre sidewalls.
It varies every 10 millimetres (example markings: 205/, 215/, 225/). The next number is the height of profile (aspect ratio), i.e. a percentage to tyre width, e.g. 55 is 55% of 225. Letters R and D indicate whether a given tyre is radial or diagonal.
The last number denotes the width of a rim in inches.