https://www.cnet.com/roadshow/news/nissan-altima-engine-mirror-bore-coating-gt-r/

Essentially, the mirror bore process replaces a traditional cylinder liner. When an engine block is made of aluminum, like the Altima's is, it requires a cylinder liner to act as a layer between the block itself and the pistons inside -- aluminum isn't capable of handling the heat and stress that comes from the pistons doing their thing, so the liner is usually made of heavier cast iron. Instead of using a liner, Nissan instead coats the cylinders with a 0.2-millimeter-thick layer of molten iron, polishing it to a mirror finish with a drill bit containing diamonds. The coating's shiny surface means less energy is lost to friction, improving the engine's efficiency.

The end result is the same -- protection of the block's cylinder walls from the forces within -- but Nissan's process brings some additional benefits to the table, as well. Considering the mirror bore procedure leaves just 0.2 millimeters of material, it makes for a much lighter solution than the usual 2-millimeter cast iron liners in use, improving overall efficiency. That thinness also makes for better heat conduction, reducing engine knock and again boosting engine efficiency.