|There is more information available on this subject at Arc welding on the English Wikipedia.|
Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas and/or an evaporating filler material. The process of arc welding is widely used because of its low capital and running costs.
There were several arc welders on board the Cradle when it performed makeshift repairs to the UNSC Iroquois and some were used by the Spartans to repair the damage to the captured Covenant flagship Ascendant Justice during a battle in slipspace, and some time later by the insurrectionist technicians at Eridanus secundus when repairing the Ascendant Justice and the UNSC Gettysburg.
In Halo canon, it is assumed that the arc welder is used for roughly the same purposes since its introduction in the late 19 century which is quite simply welding two or more pieces of ferrous metal together using an alternating current.