Explanation: The AF-area mode determines which of the 11 focus points are used. The following description is valid for taking pictures through the viewfinder, not in live view-mode.
single point: The D5100 uses one manually selected focus point that is highlighted in red in the viewfinder.
Dynamic-area AF: The D5100 uses one manually selected focus point initially but also the others around when your subject leaves the selected focus point temporarily.
“3D-tracking”: The D5100 uses one manually selected focus point initially but tries to follow the subject with all 11 focus points when it moves in the viewfinder.
This method of focus tracking uses color contrasts around the initially selected focus point.
AUTO: Automatic selection, i.e. one or several focus points will be selected by the camera electronics and highlighted in the viewfinder.
In all modes except AUTO the initially select focus point is highlighted in the viewfinder and can be moved with the multi selector.
Pressing the OK button resets the focus point selection back to the centre one.
Recommendations: For not moving subjects AUTO is a good standard setting and most convenient but watch which focus points are used! The automatic can choose background or foreground depending on the subject and it cannot read your mind and guess always correctly which part you want to have in best focus.
If you want to have best focus somewhere else you need to intervene and select a focus point manually.
And note: The exposure metering can give more weight to the area where the camera has focused. In other words: If the autofocus chooses the right distance but uses a part of the picture that is darker or brighter than your main subject you may not get the best exposure. This can be another reason to change to manual focus selection.
For moving subjects it is best to use the setting „dynamic“ and to follow your subject with a focus point. The tracking with the D5100’s 11 focus points and small gaps in-between does not work as well as with 39 or even 51 focus points that other more expensive Nikon models have. Therefore I do not use 3D with the D5100.
When using a single focus point the one in the middle works best, it is a cross-type sensor.