When cutting, the surface is affected by deformation, while the material properties closest to the surface are changed.
The geometric surface condition of the machined surface is the result of the cutting tool's design and feed rate, which gives a theoretical surface smoothness. However, this does not take into account irregularities caused by vibrations in the tool/machine, elasticity of the workpiece, edge built-up, running tool wear or variations in the position of the cutter on a milling tool.
Read more in Roughness in turning and Surface roughness in milling.
When the material in the surface and just below it deforms, a deformed harder surface results. The degree of hardening is linked to the specific material's work hardening ability.
When machining especially harder materials, microscopic geometric changes occur in the form of micro-cracks. These affect non-measurable surface smoothness but impair the material's strength as they can drastically lower the limit of crack growth.