If we are all talking about the little vertical arrow with a stalk........ from memory it points to the inbound course of the landing runway. What I cannot recall is whether it is the landing runways inbound course as found in the FMC or whether it is the course as decided by the course on the Capt's side of the MCP.
The symbol will ghost when the course is outside of the c30 degree view on the HUD, and will show you "quickest" way to turn to align to the runways inbound course.
FWIW we normally change to A III mode by now which "de-clutters" the display and provides even greater accuracy, and overall the HUD is the most commonly unserviceable item on our fleet of 30+ 737's!