More info on Tactical Items can be found on swtorista’s website. As I understand it, you do not need to worry about this until Level 75. This is part of the Play Your Way philosophy. It is used to tweak how your character, and sometimes companions, function. For tank classes this may be a shield, for example. Off-hand may be another weapon, or another item to help in combat.
Weapon Slots × 2: Main Hand and “off-hand” (i.e.Armour Slots × 7: Head, Chest, Wrists, Hands, Waist, Legs, Feet.
