Just now, I was searching for Blender Tutorial and suddenly found this video. It's an overview on Subdivision Surfaces used in 3D animation software. Of course in Blender 3D, you'll also find it.
Subdivision Surfaces has levels which make the appearance of the object you draw become smoother from one level to another. As it makes the object look smoother, it also takes up more memory and take more time for rendering. So, when you working in Draft modelling, you can use the level 1 so that it works faster. Let's watch this Blender 3D Subdivision Surfaces Introduction.