Pit and land
The depression part on the Optical track is called information pit and the flat part is called land. Pit and land are used to record information.
CD-R disc has been pressed with spiral groove, known as the "pre-groove", instead the pit of optical track. When burning, CD-R disc in a trench in an organic dye laser irradiation, the formation of bubbles, this is the "pit". Bubbles, once formed, will not be able to restore the status quo, therefore, CD-R can only be written once.