Poisoning is a special ability available ONLY to those in the Rogues' guild.
It is used via the poison command, which is used to poison your own weapon(s), or the poison <targetname> command, which is used to poison the weapon of someone else (in this case the name you replace <targetname> with).
Poisoning coats a weapon with a deadly concoction made by the Rogues' that inflicts extra damage with each attack. It also adds a chance to stun your target, as well as a chance to preform a "divine damage" (dd).