Each pilot on a Boeing 737 NG has a five-position switch labeled MAIN PANEL DUs that selects what appears on the two main display unit panels in front of her. Here's a picture of the left pilot's switch:
Most of this seems pretty obvious to me: NORM gives you the PFD (Pilot Flight Display with attitude, altitude, speed, etc. indicators) on the outboard DU and the navigational display on the inboard DU; and the three INBD settings change the inboard display to show the engine primary display (normally on the centre console upper panel), PFD, or multi-function display (normally on the centre console lower panel).
What does the OUTBD PFD setting do? In NORM mode the PFD is already on the outboard DU, so it sounds as if this wouldn't change anything.