Soldering irons melt metals to fuse and connect metal pieces to one another. A soldering iron is useful for electronics and other applications as a type of welding where the components are delicate and shouldn't be subjected to high heat.
A soldering iron is usually electric powered. You use it in conjunction with solder, a low melting point alloy that is also conductive of electricity, to produce connections useful in electronics. Fuse it right with a soldering iron.