What's "constant brightness output" or "constant current control"?

The "Constant Brightness Output" or "Constant current control" is a method to keep brightness output of headlight stable and not affect by difference of remaining battery energy.
Most of bicycle lights do not support constant brightness output, the brightness of light will be gradually changed (darker) while energy drop.
The "constant brightness output" or "Constant current control" is great feature to increase safety of bicycle riders.