the best description I've found is this.

take a teddy bear.

(Image Public Domain via Wikipedia)

now look at how they stitched the cloth together to make the bear. when UVmapping we're doing the job of making the Bear in reverse. We're taking the completed Bear and then taking it back to this :

(thank you to **www.lyrical-bears.co.uk/free.html )
**which is our VUmap.