This is a phrase coined by film critic Mark Kermode with reference to the nature of the banter that goes on between him and Simon Mayo every Friday on BBC Five Live. Reviews are often quite witty, they witter and the result is quite entertaining, hence the term. They also have a podcast version in case you miss the radio show or prefer that format.
Existing fans of the show might like to be informed that Pan's Labyrinth, the film Kermode can't stop praising (and to be fair also won an Oscar) is out and available to buy. Apparently it's part of a trilogy by Guillermo Del Toro and you're supposed to see Cronos then Devil's Backbone first to fully appreciate all three.