There is an evil bug called phylloxera. It likes to eat roots of grape vines and wiped out many vines earlier this century and late last century. However, viticulturalists realised that some vines are resistant to this bug. They planted these resistant vines in the regions that had been wiped out by phylloxera and started again. However, these vines were only of a couple of types of grapes, whereas the producers wanted to grow cabernet, shiraz and chardonnay (and all the rest) again to keep making the styles of wine that the world knows and loves. So they planted the boring vines that the bug doesn't like to eat, cut the tops (or the fruit bearing part) of the vines off the top and grafted (or spliced) the vines that they wanted on top. So now they had vines that were resistant to the plague, but producing the grapes they wanted. Grafting is the process of taking a root stock that works and placing a new type of vine on the top of it to produce whatever you want to produce without being wiped out by phylloxera.