The fork on your 700 series has a preload adjustment, as well as a compression lockout.
The compression lockout is the dial on the top right of the fork crown.
When engaged, the compression lockout decreases the low speed compression rate of the front fork, adjustable to the point of being fully rigid.
It's simply the initial compression of the internal spring in the fork. The more it's compressed, the stiffer the fork will feel. Bigger preload compresses the spring more, and so it's best for heavier riders and/or people who need/prefer the stiffer ride. Lighter riders should use less preload. The preload adjustment is is one click at a time.