On Canon camera's it is shown as Tv on your dial, but I believe on Nikons it is shown as S on your dial.

This is the shutter speed priority mode. In this mode, you control the shutter speed, and the camera controls the ISO and the aperture for a correctly exposed image. I believe that in this mode, you can also adjust the ISO if you need to.

This is a great mode to be in for sports, or anything with action, or where you would like to slow down action. In the following photo, I used a shutter speed of 1 second to expose the circles that the children were tracing with thier sparklers.