One of the benefits of using 3D cameras is that they can be animated. For example, if your scene includes a 3D object, you can animate the camera easily by moving around the 3D object to see it from all sides. When such a motion is performed on a 2D object, however, the 2D object becomes skewed and eventually turns into a line when you look at the object from the side.
You can fix this behavior by enabling the Rotate to Face Camera option in the Layer Settings dialog box. When this option is enabled, the layer is automatically rotated to stay perpendicular to the camera, so as the camera moves, the 2D layer remains flat and visible. A 2D object that faces the camera at all times is often called a sprite.