This is a great picture of the mountain fold. See how the paper stands up like a mountain ?

 Mountain fold the top half to the bottom.  A mountain fold is accomplished by turning the paper over, and then valley folding.  Make a good crease, and unfold.

 Turn your model back over, and stand the mountain up.  This is a great picture to explain a mountain fold.

Poke your finger in the middle of the paper

 Put your finger on the middle of the paper and push straight down until the corners "pop" up.

These are the first folds for the waterbomb. The dashed lines are valley folds, the solid line is a mountain fold.

The waterbomb base is the starting point for many origami models.  The frog, flowers, and a few airplaces all start with this shape.  Many model directions will simply say "fold a waterbomb base".  It is a simple base that involves two valley folds and a single mountain fold, and yields a triangle.

A valley fold is the only fold you can do with the paper flat on the ground, making the shape of a valley with the paper.