Stress is a measure of the force applied per unit of area of a cross section of the beam. Like how much force does a part of the beam “feel” passing through it.
Strain is a measure of relative deformation.
In the graphic above strain is visualized with the color scale, and you can see that the more a part of the beam has moved relative to its original state (unloaded, dark blue), the more the color of the part goes up the scale on the side.
Does that make sense? 😅
mm is the common unit used to measure rainfall.