it means for each additional inch of spring compression, it takes more weight to compress it the same amt.

so:

lets say 500 lbs makes the spring compress 1"
well to make it compress 2", it will take more than 500 additional lbs... so maybe 1250 it will compress 2"
to get the third inch down, lets say it takes 1000 more lbs... so now a total of 2250 lbs to compress the spring 3"

if it werent progressive, every 500 lbs would mean 1 more inch of compression.

hope that makes sense for ya.