You have learned to better store your equipment on your person. With each level of this Perk, the AP cost to access Inventory in combat is reduced by one.
With this perk, cost to access Inventory during combat is only 2 AP.
This perk can be very valuable for combat-oriented characters. A normal weapon reload costs 2 APs, the same as accessing your inventory with this perk. That means you can reload your gun, use stimpaks, pull out a different gun and do pretty much everything else in your inventory for only 2 action points, as opposed to simply reloading via the action bar.