Hooke's Law

 Hooke's Law states that "the extension of a helical spring is directly proportional to the weight applied, provided the elastic limit of the spring is not exceeded."

A helical spring or an elastic material and apply a weight to it it will stretch by a certain amount. If you remove the weight and apply another which is twice as heavy then the spring will stretch twice as far as it did the first time. If you remove that weight and apply a weight that is three times heavier than the first one then the spring will stretch three times further than it did the first time. This is where the "directly proportional to the weight applied" or "strain is proportional to stress" bit is relevant and importantly each time you remove a weight the spring returns to its unstressed size. However this cannot go on forever. There will come a point when the spring is stretched too far and it cannot return to its original state or in fact snaps. At this point the "elastic limit" has been exceeded and Hooke's Law no longer applies.

Definition provided by Hooke's Law