When you do 3D graphics you first build a 3D shape and then you wrap an image around it to give it surface properties like color, reflectance, and transparency.

When you do 3D graphics you first build a 3D shape and then you wrap an image around it to give it surface properties like color, reflectance, and transparency. In order to do this you need to "unwrap" the 3D shape so that it every XYZ point also exist in a 2D UV image space. This unwrapping step us usually done more or less by hand and is both tedious and time consuming.