Tyre size can be read on it's sidewall as a combination of digits.
225 - width. Tyre width is specified in millimeters. It is shown as the first value. Tyre width changes by 10 mm, for example: 175/65 R14, 185/65 R14The actual width of the tyres is usually a little bit different, than it is shown. Differences depend on manufacturer. The 225 marking means the width of the tyre section, measured on the tyre fitted to a wheel between the outermost points of the inner and outer sidewalls.
45 - tyre profile. Tyre profile is shown as the second value. Indicates the ratio of the height to the width. It is measured in percent. For example for the size 225/45 R17 the tyre height is 45% from the width 225 mm. In this case the real profile height is 101,25 mm. Tyres with the low profile have markings: 40, 45, 50. Full profile tyres usually do not have the profile value in their size.
R - means radial tyre construction. Tyres with radial construction are the most popular tyre models. Radial tyres represent 98% of total sales. In this construction type, the carcass plies are arranged at 90 degrees to the travelling direction, or radially (from the centre of the tyre). In the model 225/45 R17 carcass spreads radially from the center of the wheel.
17 - the value which informs about wheel diameter. It is specified in inches.