It is a change in position due to a force. The deformation can also be thought of as a displacement from equilibrium. Note that the restoring force is proportional to the deformation x. The restoring force is the force that brings the object back to its equilibrium position the minus sign is there because the restoring force acts in the direction opposite to the displacement.
Where x is the amount of deformation (the change in length, for example) produced by the restoring force F, and k is a constant that depends on the shape and composition of the object.
