Light sensors can be programmed to detect a certain level of light between 0 (dark) and 100 (bright). This icon waits until the light sensor has detected a level of light above a certain threshold, in this case 50.
This icon waits until the light sensor has detected a level of light below a certain threshold, also 50 in this example.
Light sensors can also be programmed in a relative way. This icon waits until the light level is 5% above the current light level.
This icon waits until the light level is 5% lower than the current light level.
There is also a light sensor fork. This program turns on motor A if the light level is above 60 (top branch) and turns on motor C if the light level is 60 or below (bottom branch).
Remember that all forks need a fork merge icon at the end of the decision.  
See also: Light Sensor, Programming: Conditionals