You're looking at a Livens Large Gallery Flame Projector in action, the mother and father of all flamethrowers, capable of torching everything as far as 100 to 130 feet away (30 to 40 meters.) They were deployed for the first time in World War I by the British Army, and apparently they were quite effective.
World War I was perhaps even more horrible and savage than World War II, with dreadful chemical weapons—like mustard gas, phosgene and chlorine—that have been banned ever since. And yet, the mere sight of these gigantic flames must have caused soldiers to run in panic and never look back.
The full-size unit you can see in action here was built by the British archeology TV program Time Team after historians from Glasgow University's Centre for Battlefield Archaeology found the remains of one of these infernal machines in Mametz, France, in 2010.
Only four of these experimental dragons were deployed in the war. Two were destroyed at the Battle of the Somme before they could be used in 1916. The other two were used near Diksmuide, and they seemed pretty effective in instilling terror in German troops: "Their use may have helped the British in those sectors of the front as British losses there were comparatively low."
This is a 3D model of the Livens Large Gallery Flame Projectors. These things were 56 feet (17 meters) long and weighed 2.5 tons.