A Rockwell hardness tester is used to determine the hardness of materials by applying a minor and major load to an indenter (a diamond cone or steel ball), measuring the depth of the indentation, and converting it into a Rockwell hardness number. This method is widely used for rapid and direct hardness testing of metals, alloys, and heat-treated components, helping to assess material suitability for various applications based on their resistance to indentation and deformation.