Soap grafting is the act of attaching an almost completely used piece of soap to a new, unused piece because it is now too small to be conveniently used but you also don't want to waste it.
To graft is to attach a twig or bud from one plant to another plant so they are joined and grow together. Here is soap grafting, it means to join one thing to another as if by grafting.
My mum always soap grafts because she doesn't want to waste anything. Soap grafting is good.