UV mapping/unwrapping is a projection of a 3D model onto a 2D map displaying all the faces of the sculpture on the map. This makes it easy to texture individual parts of a model much easier than painting directly on the model especially with, for example, straight lines wrapped around a cylinder that would be difficult to make clean by painting directly on the shape.
Learned to select the appropriate faces of the model to turn into a 2D panel without causing the model to distort.
Learned to turn on and off the checkerboard option to make it obvious which faces or parts of the model are unfinished or incorrectly unwrapped by seeing if it displays a square checkerboard pattern. If the pattern is warped or doesn’t resemble squares then there might be a problem with unwrapping.
Learned to unwrap cylindrical objects
Learned to stitch seams together that share the same edge and this makes texturing easier with less pieces to work with.
Grace's Blog 