Focus items are kind of like shields...they just have inherent energy bonuses instead of armor. You can only use focus items with 1-handed weapons, to equip it you would drag it into the slot under your weapon in your inventory window.
Correction: you should only have use for an offhand in the late game, and only if running a trapper build, in which case they can come in handy if you meet the req from your secondary. A wand/offhand combo with decent +energy and skill recharge is nice when running heavy trapping.
