What is spring-stiffness?

Answer 1

Spring stiffness is a property that relates load to deflection. Let k = stiffness and P = load and x = deflection, then

P = kx

The stiffer the spring, k, the smaller the deflection under a constant load.

k can be measured for springs and otherdevices, such as beams, with simple load deflection devices or machines that measure deflection as function of laod

Answer 2

That refers to how much force is required to pull or push the spring. The force also depends on how far you extend the spring. This is expressed via a number called the "spring constant".