The line comes from an advertisement for tourism in Australia and starred Australian actor Paul Hogan: http://www.youtube.com/watch?v=Xn_CPrCS8gs . The ad was designed for an American audience and the US English "shrimp" was used instead of the Aus English "prawn". Australians found this rather amusing and the line became well known. The line itself is simply meant to indicate that in Australia, life is relaxing and good fresh food is plentiful: enjoy yourself and cook some more food on the BBQ.

Throw another shrimp on the BBQ