Mountain-fold is one of two important folding technique in origami. Equally important folding technique is Valley-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 blue color for a mountain-fold and the red color for a valley-fold just for a convenience.
For an example, if a diagram shows a vertical dot-dashed line in the middle of the paper, it means you should mountain-fold along the line.
If you apply a mountain-fold to the dot-dashed line, the paper will be transformed to form a "mountain-shaped" figure.
The result is exactly opposite when you apply a valley-fold which produces a "valley-shaped" figure.
