What does a spot welder do?

A spot welder joins two pieces of sheet metal by clamping them together with electrodes and passing an electrical current through the metal between the electrodes. Since the metal being welded has higher electrical resistance than the electrodes, it melts. The weld quickly cools after the electricity stops flowing because the electrodes are relatively cold.

You can see spot welds on cars. Look for dimples near the edge of two plates or panels. They are easier to spot on the undercarriage, inside the trunk and on some frame members especially where bodywork and trim doesn't hide them.