Valley-fold is one of two important folding technique in origami. Equally important folding technique is Mountain-fold. If you know these two techniques, then you can fold almost all of simple origami models.
When you are studying an origami diagram, you will be encountered with line symbols such as dashed line or dot-dashed line.
A simple dashed line represents a valley-fold while a dot-dashed line for a mountain-fold.
Here I use the red color for a valley-fold and the blue color for a mountain-fold just for a convenience.
For an example, if a diagram shows a vertical dash line in the middle of the paper, it means you should valley-fold along the dash line.
If you apply a valley-fold to the dash line, the paper will be transformed to form a "valley-shaped" or "V-shaped" figure.
The result is exactly opposite when you apply a mountain-fold which produces a "mountain-shaped" figure.
