How I understand it is an LOD contains a mipmap of the visuals (meshes, materials, particles, etc) at every level defined in the LODs for those visuals. A mipmap is basically a preset resolution and render quality of the final image, or image before the final. It reduces draw calls and other memory usage / processor using render ‘types’. It’s in the doc pages that the power-of-2 rule is required for LODs / mipmaps, but I don’t know what happens if a texture is not a power of 2 size. And the question I have, among others:
Is the size (in reference to power of 2) of a texture the pixel resolution? or another sizing attribute?
So, as far as I understand it in simple terms:
LOD 0 is the lowest level, having the highest resolution (by default)
LODs 1-n (n being number of LOD levels set and defined via settings) are sequentially lower resolution
Each LOD level maintains a mipmap specifically generated per LOD (possibly more than 1 per LOD too)