Exposure Ramping is a method by which the shutter speed (or the duration of exposure) and/or ISO (how sensitive your camera is to light) change(s) over the course of your time lapse. This allows you to capture properly-exposed time lapses, even during situations with gradually changing light levels, like a sunset or sunrise.
The 'Holy Grail' of Time Lapse is a full day-to-night, or night-to-day transition, and it can create some incredible effects (just see below). However, it is nearly impossible to achieve through changing the shutter speed alone because of the huge changes in light levels involved. In order to shoot a Holy Grail timelapse, you will almost certainly need to adjust your camera's ISO and/or Aperture along with the shutter speed.