The Red Dwarf crew intended to deliver an escape pod from a prison ship, as it either contained a female guard or a bio-mechanical killer. If the vessel has no clearance code, they are required to state "lifeform inventory" to the Justice Computer and to submit navigation to its jurisdiction. They are then require to proceed through the Neutral Area to the clearance zone.
This is where visitors wishing to enter the penal colony without a clearance code must pass through to gain entry. They are then met by escort boots and are required to step into them (they likely contain hologram emitters, otherwise Arnold Rimmer wouldn't be able to step into them). They make the visitors walk towards the mind probe to search their minds for any evidence of guilt of criminal activity that they may have been hiding (not counting crimes for which they have served the punishment) and if none is found, they are allowed to step out of the escort boots. If a crime is detected (with the guilty effectively convicting themselves), they are escorted to the Justice Zone, the main prison area covered by a Justice Field that prevents anyone from physically committing any crime.
The cells have no locks or bars, because the Justice Zone makes them unnecessary. The main prison area has shopping malls, parks and entertainment facilities. Deleted scenes for the episode show that the greenery inside the "hanging scales" are botanical gardens, and there is scene whereby Lister walks in them (in a scene filmed in a London park) and Lister drops litter and is pooed on by a giant bird in retaliation. The reason for this is because the people who came up with the idea of the Justice World penal colony believed that rehabilitation of criminals isn't possible by taking them out of society, so to solve that problem, a self-contained society is built, one where crime is physically impossible.
The justice field is an energy field that covers the Justice zone area of the colony.
The field does not in fact make committing a crime physically impossible as those within it at the time can still attempt to do unlawful acts, however any crime committed while under the justice field will be reflected back onto the one committing it. Trying to set a fire will result in you bursting into flame, attempting to steal something will have something of your stolen instead, injuries from assault are reflected back onto the attackers and so on. By the time the prisoner is released not committing crime is second nature to them.
Behind the Scenes
An interview on the official Red Dwarf website suggested that Ackerman, warden of The Tank on Red Dwarf, used to work at Justice World, alongside Barbra Bellini, whom he was having an affair with and had to leave in a hurry when caught out. Ackerman was the one who accidentally set the simulants free; providing an interesting irony that he later imprisoned those who helped contain one of those he had set free.