The "L" is a 2nd Arca-Swiss base plate rail with a 90 degree bend that can be attached into (or removed from) the main camera arca-swiss base plate. Now you can turn the camera sideways into portrait mode and the L Arca-swiss plate is attached to the tripods Arca-swiss clamp.
So your camera goes on a tripod in normal landscape or now also portrait mode.
my tripod is not arca capable? (manfroto).. never used it.. does that site sell an adapter cheap? what's the advantage.. heard many people talk about it, but haven't seen one in use