a1 AF-C Priority Selection

Explanation: This menu defines when you can release the shutter and actually take a picture while the D5100 is set to continuous autofocus AF-C (see focus mode):

• Release (default): You can release the shutter anytime, no matter if focusing is accurate or not
• Focus: You can release the shutter only when the autofocus confirms correct focusing.

The advantage of „release“ is that you have no delay when pushing the shutter release button – but possibly pictures out of focus. With „Focus“ it is the other way round – pictures will be in focus but the shutter release may be delayed.

Recommendation: I prefer focus priority. With release priority you can expect unsharp images when shooting moving subjects.