By default, the portables with ambient light sensors (ALS) automatically adjust the brightness of the keyboard backlight based on the ambient light in the environment. The ALS, located to the left of the FaceTime camera, evaluates the level of light for this feature. While in the default mode, the F5 and F6 keys will function only while the light levels are low enough for the ALS to trigger the backlight keyboard. By covering the ALS, you can test the backlight keyboard feature and adjust it with the F5 and F6 keys.
The default setting for portable computers, as seen in OS X Lion:
Note: Image may vary slightly with earlier versions of Mac OS X. If you do not see the "Automatically illuminate keyboard in low light" option, your computer does not feature a backlit keyboard.
You can also turn off the automatic backlighting feature in the Keyboard preferences. Deselect the option "Automatically illuminate keyboard in low light" to use the F5 and F6 keys to adjust the keyboard backlight independent of ambient lighting.