is the format.
e.g, my car has Durun D103 145/70R12, means:
- 145mm tread width,
- sidewall is 70% of the tread width, (If no profile rating mentioned then it is 80%)
- R means radial (if the tyre is not radial, a dash "-" is present)
- 12 (in inches) is the rim diameter (or the internal side of the tyre)
This scheme is according to USDOT.
Some other letter also tell the speed rating e.g. 155SR13 (usually installed in Margalla) tells you:
- 155mm treadwidth
- By default 80% profile
- SR means radial w/ 180kmph speed rated
- 13 (inch) wheel dia.
If you go into ZR rubber, designated for over 249kmph, there are other letters too, but that is too much detail.
(Faster speed means softer rubber with more grip and more safety but also greater treadwear). There are other letters such as HR, TR, I don't remember their speed rating, you can actually search on the net for it.
The other important thing is load capacity.