R= Radial
3 digit # is tire width in millimeters.such as 195 or 225 etc
T, S, V, H, etc is tire speed rating( how fast a tire has tested to and still rated as safe)
75, 65,60, 55, 50 40, 30 etc is tire aspect ratio. the comparison of tire width to tire side wall. it is a percentage, such as 50%, 60% etc. The lower the # the wider the tire in respect to wall height.
14, 15, 16, 17, 18, 19, 20, 22 etc is wheel diameter (measured at bead seat, not at very edge of wheel.
there will be other #s in smaller print which rate tire wear rating(how long a tire is rated to wear such as 400 or 450 etc)
also letter rating related to traction, heat wand wear , these are A,B,C
and of course tire pressure recommendation