A metal is tempered by heating it below its austenitizing temperature. This treatment decreases the residual tension inside metal and therefore makes it less brittle.
Depending from the tempering temperature the result will be different: higher temperatures will improve toughness, lower temperatures will improve hardness.
Usually tempering is performed after quenching.
Tempering helps to reduce internal tensions of the material, improves structural stability and increases toughness, but reduces hardness and mechanical resistance.