Origami Valley Fold

Any model of origami starts with either a valley fold or a mountain fold. 

Valley Fold

In the origami diagrams, the valley fold is represented by "-------"

Start with a origami paper, then make a fold from either left to right or from right to left. This is known as a valley fold.
Once the paper is folded with the valley fold and unfolded we see that the paper forms a V.

Let us look at the images to understand it much better.