Manual Control M
Explanation: You choose both an aperture and an exposure time manually; the exposure time with the command dial and the aperture with the exposure compensation button and the command dial.
The D5100 will only give you an indicator in the view finder if the resulting exposure is brighter or darker than the one measured by the camera.
Note: The auto ISO sensitivity control from the shooting menu ISO sensitivity settings is not deactivated automatically when choosing mode M as it would be appropriate for the purpose of full manual control. If you do not switch of the auto ISO sensitivity control yourself the D5100 will move ISO sensitivity up and down and you still have some kind of automatic exposure control.
The display „bulb“ for the exposure time means that the shutter will be open as long as you hold the shutter release button pressed.
Recommendation: This mode is for usage in specific situations – those who use it know why, if you need to ask first when you should choose this mode just stay with the other modes.