Resistance spot welding got its name because the base metal's resistance to current flow causes a localized melting in a part. Heat is applied to a material, pressure is exerted by electrodes and tips, and the amount of time needed is determined by a metals thickness/type, amount of current, and the cross-sectional area of the welding tips and contact surfaces.
Resistance spot welding happens when current flows through the tips and the separate pieces of metal to be joined. The resistance of the base material to the electrical current creates localized heating and the weld is made.
The weld is unique because the weld nugget is formed internally in relation to the surface of the base metal.
For comparison, an arc spot weld starts at the surface of one piece and penetrates into the second piece to form the weld nugget. The arc weld is made from one side vs a resistance spot weld that is typically made with electrodes that press both sides of the workpieces.