The Normal input takes in a normal map, which is used to provide significant physical detail to the surface by perturbing the "normal," or facing direction, of each individual pixel.
In the image above, both weapons are using the same Static Mesh. The lower one exhibits a highly detailed normal map, which provides additional detail, giving the illusion that the surface contains many more polygons than are actually being rendered. Typically, normal maps are often created from high-resolution modeling packages.
