All these numbers combine together to give you the width, profile, rim size, load and speed rating.
235= Over all tyre width in millimetres
45= The aspect ratio or profile of your tyre. This determines the sidewall height of your tyre as a percentage of the tyre width. In this case, the height of the sidewall is 45% of 235mm
R= Radial construction
17= Rim diameter in inches
94= Load rating index. How much load each tyre can carry. A tyre with a 94 load rating can carry 670kg
W= Speed symbol category. A W rated tyre has been tested as being safe and strong enough to maintain 270 km/h