Soldering is a process that joins two or more metal items together by melting and flowing a filler metal called solder into the joint. A soldering iron is used to heat the metal parts while solder is applied. Solder has a low melting point between 90 to 450 degrees Celsius, allowing it to flow and bind the parts together electrically and securely. The key steps are preparing materials like a soldering iron, solder and circuit board, heating the joint with the iron, and applying solder so that it flows smoothly into a volcano shape between the joining pieces.