Mip Maps are a list of progressively smaller versions of an image, used to optimize performance on real-time 3D engines. Objects that are far away from the camera use the smaller texture versions. Using Mip Maps uses 33% more memory, but not using them can be a huge performance loss. You should always use Mip Maps for in-game textures; the only exceptions are textures that will never be minified (e.g. GUI textures).

We base our definition of Mip Maps directly from Unity. 

For more information Please visit https://docs.unity3d.com/Manual/class-TextureImporter.html