Light in Minecraft affects certain things, such as mob spawning and visibility. Light can be provided by Ambrosium Torches, Aether Portals, sunlight, fire, and various other sources.
There are 16 total light levels in integers, which are 0 to 15. Minecraft uses the light levels of blocks to calculate the brightness of a selected block.
If the light level is 0, a block will be close to black. If the light level is 15, a white block will be close to white, and a green block will be close to green.