The function keys.
function
On most laptops - hold down the fn key (next to the CTRL key) - and press function keys f6 or f7 to adjust the brightness. Release the keys when you are happy with the level. The laptop will retain the setting until you change it (even on power off).i also have a problem with that but also there is no brightness adapter in the power options sittings!
Most laptops can switch between the internal display, the external display, and both by pressing the "Fn" key and one of the function keys (such as F5).
Hold down the Fn key (next to the CTRL key) - and press either F6 or F7 to change the brightness. Release both keys when you have the desired level. The screen will retain the level until you want to change it again.
Dell and Acer
Your question wording is confusing, but I will attempt to guess what you mean and answer it. Some (but not all) keyboards have a backlight to help you identify the keys in low lighting conditions. On such keyboards a pair of keys (usually multifunction "f keys") are used to adjust the brightness of this backlight to meet the user's needs. That is the purpose of the key on the keyboard that you are asking about.
Just Depress the reset button on the back and the display will show the strength. Then use the arrow keys to increase or decrease
Fn, or 'Function,' is a modifier key on manykeyboards, especially on laptops, used in a compactlayout to combine keys which are usually kept separate. It is typically found on laptops, since a full sized keyboard would be difficult to fit in a laptop chassis. It is also found in many full-sized 'multimedia' and 'office' keyboards, named F Lock key. It is mainly for the purpose of changing display or audio settings quickly, such as brightness, contrast, or volume, and is held down in conjunction with the appropriate key to change the settings.
you push the fn key next to the Ctrl/control key. You hold down that button and push the arrow key up for the brightness to go up and the down key for it to go down. If the brightness doesn't go up then its probably on full brightness.
all laptops are different, just count them
The iMac keyboard has brightness keys - on the current model on the F1 and F2 keys. The screen can be calibrated by selecting Displays within System Preferences, then clicking the Color button and the Calibrate... button. This will talk you through the screen calibration.
If there is a BIOS password or an automatic brightness program active in the BIOS then your brightness keys may not work.