Solder is a metal alloy commonly comprised of tin or lead that melts when heated and allows a permanent connection to be made between electronic parts or wires. The picture gives an example of someone using a solder gun and soldering a circuit board.
Electronics terms
When soldering make sure you are in a well ventilated room, have a fan, or have some type of fume extractor. Breathing in the fumes generated by melted solder can pose a health risk from the rosin and other elements.
