The "flag" houses a holographic scanner and a range-finding sensor which interface with a HUD inside the helmet. The entire whole unit is typically referred to as a 'rangefinder'
Targeting rangefinder (retracted) The helmet tracking sensors sent information directly to the helmet displays. It gave Fett precise readings on the speed of, and distance to any given target.
and
Holographic targeting display When flipped down, the targeting display interfaces with the visor to give tracking information on up to 30 targets at a time.
From Empire Strikes Back, as Fett is watching the Millennium Falcon emerge from hiding amongst the Star Destroyer's garbage after fleeing Hoth.
