A picture, or a graph, is worth a thousand words somebody said.  So here is an image showing the difference in string stiffness.

Simply, the shorter the distance (mm) to 50 pounds the stiffer the string.  So you see one string (blue) reached a force of 50 lbs at 26.0 mm, another (red) at 27.9  mm, and another (green) at 49.6 mm.

Soft v Stiff String

Another property we see is linearity, or how straight the line is from beginning to end.  This could indicate the consistency of the string through various hitting forces.

Ultimate tensile strength (UTS) is indicated by the line going straight down because the string broke at that force.  The force and distance are displayed near that vertical line.

The two top strings are polyester and the lower one is PEEK.  These just happened to be strings installed in racquets today.