Can someone please explain to me what these spring rates mean . They are to do with 444 Series Progressive rear shocks . The standard spring rate is 90/130 lbs/in , 1.61/2.32 kg/mm , heavy duty spring 105/150 lbs/in , 1.88/2.68 kg/mm . Thanks in advance .

It must be a dual rate spring.
The measure is the amount of weight to compress the spring by 1 inch ( or 1mm -metric).
e.g. a 90lbs/in spring will compress 1 inch for every 90 lbs of weight placed on it.