Puts the camera in orthographic view mode.
Orthographic is (according to Wikipedia):
... a means of representing a three-dimensional object in two dimensions...
Bascially, Orthographic is a fixed depth. You can't zoom in or out. There is no distance from the camera.
Perspective has depth. You can zoom in and out. You can judge distances. Cameras can be distanced from the scene.
Simply put, orthographic is what would happen if you squished everything in the scene (along the cameras view axis)
and stuck it right against the camera.
Perspective on the left, Orthographic on the right