If you're thinking of the King of Pentacles, for example, the grapes represent the sweetness of life he is able to enjoy. He has put a lot of effort into things and is now able to enjoy the fruits of his labour.
I'm pretty sure there is a reference to the Eucharist in there somewhere as well, the RWS being a Christian deck, and to the mysteries of Dionysos, from which the Christian rite seems to be partly derived.
