All Optical media (CD, DVD, Blu-Ray etc.) have pits and lands. These are microscopic and represent the binary information of the data stored on the disc. A land is reflective and reflects the laser into a sensor to register it as a I, but when the light hits a pit, it shatters and no reflection is received, thus a O is registered.
To write 1’ & 0’s on CD, a laser beam is used. To write 1, the laser beam is turned on, which turns a pit up to the reflecting layer. To write 0, the laser beam is not turned on & hence, no pit is burned. The surface when there is no pit is called land.