In arc welding, you build up a high voltage between an electrode and your work surface, which consists of two separate pieces of metal. When you bring the electrode near the parts you want to weld, this high voltage creates a spark, which is hot enough to melt some of the metal from the two parts - it's basically the same as lightning! Often, the electrode itself or a separate piece of filler metal is also melted, which adds some material to help join the parts. The melted metal mixes and cools, forming a single solid part.
I hope that helps!