6.0, 7.0, CS, CS2
An edge is formed where adjacent pixels have high contrasting values.
Photoshop has a number of filters that detect these and apply contrast reducing or increasing effects to soften or sharpen the image accordingly.
The top half of the picture (2) is an enlarged part of the stem of the plant in the picture above (1).
The Find Edges filter picks up all the areas of edge contrast ...