S (Shutter Priority)
Explanation: Shutter priority means that you choose an exposure time (shutter speed) and the D7000 will choose a corresponding aperture.
The exposure time can be selected with the command dial at the back.
If you use the flash you cannot select any shorter exposure times than 1/250 s (except when using the so called FP high speed sync from menu e1 Flash sync speed and an external flash).
If there is no aperture for your exposure time that results in correct exposure there will be an indicator for over- or underexposure in the viewfinder.
Tips: The most important usage of shutter priority S is to select a short exposure time to capture fast moving subjects.
But you can get the same results in mode P and especially the recommended modes U1 and U2 by turning the command dial at the back to shift the combination of aperture and exposure time.