What is force constant?

Hint: Force constant is also called spring constant, we will start by understanding the law that brings force constant as a proportionality constant. This law is called hooke's law that brings about the relation between force applied and the distance stretched in the spring.

Complete step-by-step solution:
You might have observed the movement of springs many times, this movement is called SHM( simple harmonic motion) . You know that once you apply force to the spring it either compresses or stretches, but this stays only until the force is applied. As soon as the force applied is removed, it comes back to its original state .Hence it is said to be a SHM.
  After the force applied is removed it remains stationary. We know it comes back to its original position. Do you know why? It is because it has a restoring force according to Newton’s 3rd law.
Hooke stated that force required to compress or stretch a spring is directly proportional to the distance to which it is stretched or compressed. Thus combining Newton’s and Hooke’s law we find that :
F\[ \propto \]x
Where; ‘x’ is the distance to which the spring stretched/compressed.
F is the restoring force,
F=-Kx; -------(1)
Where K is the proportionality constant called spring constant or force constant.
Hence spring constant/ force constant is defined as force per unit of extension or compression of the spring.

Note: The sign in the equation is negative because force is opposite to the direction of applied force.it is the restoring force.
The SI unit of force constant is N/m.
Spring constant is inversely proportional to distance travelled by the spring during its compression or stretch.