I spring is a material (usually a metal, but could be a plastic as well) that is under stress, typically tension, torsion or compression (depending on type of spring). The limit is basically the yield strength of the material, which is the point where the stress vs strain curve goes nonlinear and the material begins to plastically deform. Obviously the fracture limit represents catastrophic failure of the material, but that is higher than the yield limit.

Basically, it's a material strength and elasticity problem.