These Blocks, Mobs, or Structures emit a light level of 2-15. The light level of the emitter signifies the light level at the block, and the surrounding blocks has decreasing level according to the distance. I.e. Given a dark area lit by a single Torch (light level 14), the light level at 2 blocks to the North, 3 blocks to the East and 1 block down will be 14 - 2 - 3 - 1 = 8. Monsters are able to spawn when in light levels 0 through 7.