Weapon handling is used by the NCTH calculation to modify hit chances based on how cumbersome a weapon is. The idea here is that small, light weight weapons will be easier to control and 'stabilize' then large, heavy weapons.
Originally, NCTH didn't have a Handling tag. Instead, it used the ubReadyTime to determine the handling modifier. I added the Handling tag to give modders more control over weapon handling and to resolve a problem where ubReadyTime=0 weapons were getting no handling modifiers at all. I never got around to adding information for this tag to the UDB interface. I'll add that to my todo list.