Springs can be used to store elastic potential energy.

E.g. if you pull back a NERF gun, energy is stored in the spring. When the gun is fired, the elastic potential energy is transferred to kinetic energy and heat.

Some springs are stiffer and harder to stretch than others. The stiffness of a spring is called its 'spring constant' and is measured in:
newtons per metre         N/m

The spring constant is the force needed to extend the spring by 1m.

e.g. 6.55 N is needed to extend a spring 10cm. So the spring constant is 65.5 N/m

The greater the spring constant, the stiffer the spring is.