To demonstrate what happens to a body (mine, for example) when a force is applied, I stand on something to reduce friction: skates or a skate board. Then somebody pulls me with the spring scale, ...

The spring constant of a spring can be found by carrying out an experiment. The unloaded length of a spring is measured. Slotted masses are added to the spring. Record each stretching force in N ...

a graph of force against extension produces a straight line that passes through the origin. The gradient of the line is the spring constant, k. The greater the value of k, the stiffer the spring.