Some digital cameras have an orientation sensor so that photos shot in portrait mode are automatically rotated right-side-up when you look at them. However, since some digital cameras do not have such a sensor, photos shot in portrait mode appear sideways in the Photo Browser. This can be corrected by rotating them.
Note: Rotating either raw or TIFF files does not modify them in any way; instead, the rotation information is stored in sidecar XMP files. Rotating JPEG files modifies only the rotation metadata: the image data is not modified.