When combining multiple images, it is critical that each image be captured from the same point of view. To achieve this, the camera must be rotated about the "no-parallax point" of the lens, which is the the center of the lens's entrance pupil, a virtual aperture somewhere within the lens.1,2 This point is often incorrectly called the "nodal point", but is unrelated to the actual nodal points of the lens. In the wide angle lenses used for panoramic imaging, the entrance pupil tends to be near the front of the lens.
A standard tripod mount rotates the camera around the mounting screw in the camera; the simplest so-called nodal point adapter simply shifts the camera back to move the lens's no-parallax point over the axis of rotation. More complicated brackets allow the camera to rotate vertically as well as horizontally around the no-parallax point.
A database of entrance pupil measurements has been set up, where you can find the measurements for your combination of camera, lens and focal Length. If your entry is not listed, you can find the entrance pupil for your combination. The links section of the No-parallax point page lists a number of external sites describing the procedures.