LBS per Inch. Basicly it takes so many pounds of force to move a coils spring 1 inch.
For example. 600lbs/in. It takes 600lbs to move the spring 1 inch in travel.
So as your traveling down the road the weight of the bike and you and the force applied by a bump in the road is held by that spring. So there needs to be a force that will ubsorbe that force and weight without letting the bike compress so far that it bottoms out. There's some math involved, but that's the basic idea.