What is a choke chain?
A choke chain is what is suggests; a loop of chain that goes around a dogs neck. It tightens when a dog pulls and is primarily used to 'correct' leash pulling issues.
It is a form of positive punishment and there is no other way of putting it. It is cruel. As a dog owner who uses positive reinforcement I find it completely useless.
An Association of Pet Dog Trainers UK leaflet on the matter can be found here. I encourage you all to read it.