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.