Multifiring is the ability to fire more than one of the same weapon at once. Berserkers and Gunners enjoy this benefit inherently, but there are a few items that allow others to multifire, such as the multifire controlbot. The effect "+100% Multifire" allows you one more weapon you are able to fire.
It is interesting to note that a long while back, anyone could use several weapons simultaneously with multifire; however, this was changed so that it is now only a class skill.
Should not be confused with the multi-bullet feature.