What do all the indicators on a HUD mean?
HUD symbology varies depending on flight mode and aircraft configuration so you will find a wide variety of symbols capable of being displayed.  Some basic ones from the F/A-18 E/F aircraft are.



To your original question the ‘W’ symbol is called the Waterline symbol and represents the point at infinity where the longitudinal axis of the aircraft points toward.  It is analogous to the miniature airplane symbol on an attitude indicator.  A similar symbol called the Gun Cross is used on the F-16 HUD.

On high performance aircraft capable of high AoA operations, the waterline symbol is not as useful as the Flight Path Marker or Velocity Vector symbol which indicates the point at infinity the aircraft is currently trending towards.