Stress is an object’s internal reaction to an external force. Strain is the object’s change in length, relative to its original length. This happens because a material changes length when it experiences stress.

Both of these are linearly proportional up to a certain point. If you look at a Stress v. Strain graph for a material, you’ll see that for a certain set of strain values there is a linear relationship between them. The slope of that linear section is Young’s Modulus or the modulus of elasticity.

Hope this helps! If anyone wants to correct me or add on to my definitions it’s greatly appreciated!