This “iron monstrosity with a hatch”, described on the page as being like “an iron coffin” is the time machine from the film. To put the role of a time machine in context, here’s how Jonson explained the film’s plotline.
Looper is a time travel movie, set in a near future where time travel doesn’t exist but will be invented in a few decades. It’s pretty dark in tone, much different from [The Brothers] Bloom, and involves a group of killers (called Loopers) who work for a crime syndicate in the future.
Their bosses send their targets hogtied and blindfolded back in time to the Loopers, and their job is to simply shoot them in the head and dispose of the body. So the target vanishes from the future and the Loopers dispose of a corpse that doesn’t technically exist, a very clean system.