Cutout shader is an alternative way of displaying transparent objects. Differences between Cutout and regular Transparent shaders are:
This shader uses an alpha channel contained in the Base Texture to determine the transparent areas. If the alpha contains a blend between transparent and opaque areas, you can manually determine the cutoff point for the which areas will be shown. You change this cutoff by adjusting the Alpha Cutoff slider.
This shader is Vertex-Lit, which is one of the simplest shaders. All lights shining on it are rendered in a single pass and calculated at vertices only.
Because it is vertex-lit, it won’t display any pixel-based rendering effects, such as light cookies, normal mapping, or shadows. This shader is also much more sensitive to tesselation of the models. If you put a point light very close to a cube using this shader, the light will only be calculated at the corners. Pixel-lit shaders are much more effective at creating a nice round highlight, independent of tesselation. If that’s an effect you want, you may consider using a pixel-lit shader or increase tesselation of the objects instead.
Generally, this shader is very cheap to render. For more details, please view the Shader Peformance page.