Explanation: You choose both an aperture and an exposure time with the two command dials.
The D7000 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 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 D7000 will move ISO sensitivity up and down and you still have some kind of automatic exposure control.
Tips: 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 😉 .