A soldering iron is a hand tool used to heat and melt solder, which is a metal alloy used to join electronic components to circuit boards or wires. It typically consists of a metal tip that heats up when powered, allowing it to melt the solder. Soldering irons are commonly used in electronics, electrical repairs, and craft projects to create strong, conductive connections between components. They come in various sizes and power ratings, depending on the specific application and type of work required.