The first 0 KM point of the Tour de France 2007
After the fictive departure at the start arch, each Tour de France stage has a so called "0 kilometers point" where the stage really starts. Before they arrive at this point, the cyclist must stay behind the official Tour de France car and they can only make a jump to escape from this point.
For the first stage from London to Canterbury this point was very symbolically chosen to be in Greenwich, the reference point for the Greenwich Mean Time (GMT).
During the Tour everyone uses the 0 KM point as a reference to indicate where you are on the track of the stage: if you want to say that you're at 88 kilometers after this point for example (that day that would have meant "I'm in Tonbridge"), you say "je suis au PK 88" (PK for "point kilométrique").