They're talking about the spring rate... i.e., how many kilograms of force will compress a given spring one millimeter.

For comparison, the Tein coilovers I use consist of a 8kg/mm spring in front and a 6kg/mm spring out back.
I would consider that the limit for a daily driven DC Integra that doesn't see much track time.

Also, you need to match the damping rate of your shocks to the spring rate you choose to run.
As you shop for dampers, research the recommended spring rate for the model you are considering.

Also, read through the suspension section of the Integra FAQ, if you haven't already done so.