The slope of the line is the spring constant (k), which characterizes the stiffness of the material.
The relationship between the force applied to an elastic material and the subsequent extension or deformation of that material is commonly represented by a force against extension (or force against displacement) graph.
A graph like this is frequently connected to Hooke's Law, which states that the force needed to compress or extend a spring is directly proportional to how far the spring has moved from its equilibrium position.