Rupert is nickname for Officers in the British Army, and these Ruperts were all 3ft tall dummies. They were used to distract the Germans whilst troops landed elsewhere. And they did so successfully! A whole unit of German troops were convinced they were watching troops parachute in and start firing guns. In fact, they were hearing firecrackers explode as the dummies hit the ground.
There was actually a whole bunch of fake operations to distract the Germans as well as a fictitious US army unit called the US First Army group. It was complete with inflatable tanks and fake radio traffic.
Thanks to QI on BBC two for this one.