Wield, to use (a weapon, instrument, etc.) effectively; handle or employ actively.
Most powers will have an implement or weapon keyword, thus will benefit from using an implement, or require the use of a weapon, either ranged or melee. It is possible to dual-wield but your attacks will only be made through the weapon/implement of your choice, usually your main hand. There are feats however, like Dual Implement Spellcaster that allows you do use two implements for one power. Also, some powers, like a dual-wield ranger build, will require a melee weapon in each hand.
Sometimes, you will wield a weapon/implement in your offhand just for the magical benefit. For example, some magical items give an enhancement bonus to initiative while others can add extra power to your healing.