Horizontal orientation reffers to photographs which are wider than they are tall.

"Horizontal photographs is the most common photographic orientation. Horizontal images are roughly the same format as how we see the world and have some distinct properties. Horizontal frames can depict a wider perspective. Horizontal images are best used:

When the subject is horizontal

When your subject is wider than it is tall

When the subject moves horizontally

When subject crosses from one side-side of the frame

A horizontal format in conjunction with the rule of thirds visually allows the subject room to continue moving. This will amplify the sense of motion in the image. This also helps to superimpose on the gaze of the subject, giving completeness to the image. Horizontal frames are also used to photograph landscapes giving them depth and detail."