Explanation: In some situations the question mark above the zoom out button applies and pushing this button will display help texts.
This is the case
- in camera menus when see a little question mark at the bottom left
- in the shooting information
- sometimes while taking photos when a question mark is blinking at the bottom left of the monitor.
Then the D5100 wants to give a tip, e.g. to use the flash.
Recommendation: If you need the help texts depends on how well you know the camera already and if you like their style. Just give it a try.
I would not follow blindly the regular tip to use the flash, the D5100 is very generous in showing it.