90 is the width in millimetres of the tyres cross section.
100 is the Aspect Ratio, it is the ratio of the sidewall height to the cross section width.
10 inches, stands for the diameter of the wheel that the tyre is designed to fit.
Radial construction.R stands for radial-ply construction.
0 is the load index i.e. the maximum weight each tyre can carry at full speed.