R for Rear Curtain
Rear Curtain is a term used to describe flash setting when the flash is fired at the end of the exposure. In general, the flash is fired at the beginning. In a fast shutter speed, it doesnt really matter, but in the slower ones, having a rear curtain flash has very interesting effects. It shows movement, and then freezes at the end moment. On the flip side, is the front curtain, which also shows movement, but in a different form. I suggest all to try these out. A full detail post can be found here.